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ELEMENTARY  PHYSICAL  GEOGRAPHY 


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Plate  1,  —  Frontispiece!. 
Watkins  Glen,  N.Y.     A  post-glacial  gorge  in  a  shale  rock. 


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ELEMENTARY 


PHYSICAL  GEOGRAPHY 


BY 


RALPH   S.   TARR,  B.S.,  F.G.S.A. 

ASSISTANT   PROFESSOR  OF  DYNAMIC  GEOLOGY  AND   PHYSICAL 

GEOGRAPHY   AT   CORNELL   UNIVERSITY 
AUTHOR  OF   "ECONOMIC   GEOLOGY  OF  THE  UNITED  STATES" 


MACMILLAN    AND    CO. 

AND   LONDON 

1896 

All  rights  reserved 


Copyright,  1895, 
By  MACMILLAN  AND  CO. 


Set  up  and  electrotyped  October,  1895.     Reprinted  March, 
1896. 

EDUCATION  OB^^ 


Norinooti  l&xtii 

J.  S.  Gushing  &  Co.  —  Berwick  &  Smith 
Norwood  Mass.  U.S.A. 


PREFACE. 

For  some  time  there  have  been  indications  that  new  text- 
books on  physical  geography  are  demanded ;  and  in  the 
report  of  the  Committee  of  Ten  this  finds  definite  expression. 
In  the  preparation  of  this  book,  which  has  been  in  hand  for 
several  years,  there  is  an  attempt  to  meet  this  apparent 
demand;  but  for  reasons  which  are  obvious  to  many,  it  has 
not  seemed  wise  to  attempt  to  follow  the  somewhat  radical 
suggestions  which  were  made  by  the  majority  of  the  geogra- 
phy conference  of  the  Committee  of  Ten.  Therefore,  while 
the  physiographic  side  is  given  more  prominence  than  is 
customary  in  works  of  this  kind,  this  book  attempts  to  only 
partly  meet  the  Committee's  suggestions. 

In  the  preparation  of  the  book,  effort  has  been  made  to 
introduce  new  material,  particularly  in  the  illustrations, 
which  are  a  prominent  part  of  the  book.  Also,  there  has 
been  an  endeavor  to  make  the  book  scientifically  accurate, 
and  to  introduce  the  latest  knowledge  on  the  subjects 
treated.  There  are  probably  places  in  which  this  is  not 
done,  for  the  field  is  so  large  that  much  must  be  compilation ; 
and  the  compiler  is  liable  to  fall  into  error. 

I  anticipate  criticism  of  the  order  of  presentation,  of  the 

relative  amount  of  space  allotted  the  various  topics,  of  the 

vii 

5^4297 


*  •  • 


Vlll  PREFACE. 

omission  of  some  subjects  which  are  usually  found  in  such 
books,  and  of  the  inclusion  of  some  not  usually  discussed ; 
but  these  matters  have  been  carefully  considered,  and  the 
book  is  the  result  of  a  well-matured  plan.  In  many  respects 
it  is  experimental,  but  it  is  a  deliberate  attempt  to  supply  a 
book  which  is  certainly  needed.  It  should  not  be  inferred 
that  the  author  is  satisfied  with  the  attempt, — he  is  keenly 
disappointed  at  the  constant  necessity  of  saving  space  and 
thereby  weakening  description  and  explanation.  In  many 
cases,  explanations  have  been  omitted ;  in  others,  perhaps  it 
would  have  been  better  to  have  done  so. 

It  is  hoped  that  the  more  advanced  teachers  will  find  it 
possible  to  accompany  the  text-book  work  with  laboratory 
and  field  study,  along  the  line  suggested  in  the  appendix. 
The  discussion  of  method  has  been  systematically  eliminated 
from  the  text,  and  the  sole  effort  has  been  to  present  facts 
and  furnish  information ;  but  if  this  alone  is  put  before  the 
pupils,  the  value  of  the  study  will  be  very  slight  indeed.  It 
furnishes  the  main  story  in  a  connected  way,  and  supplies 
certain  information  ;  but  the  laboratory  and  field  will  supply 
applications  and  extensions  of  the  principles,  at  the  same 
time  giving  value  to  the  study  as  a  means  of  mental  disci- 
pline. Merely  to  hear  recitations  from  the  book,  will  be  the 
continuation  of  an  all  too  prevalent  habit,  which  in  so  many 
cases  makes  the  science  teaching  in  our  secondary  schools 
the  weakest  part  of  the  curriculum. 

While  the  author  has  done  much  work  in  some  of  the 
subjects  treated,  particularly  the  ocean  and  the  land,  he 
would  not  wish  to  claim  that  much  in  the  book  is  original. 
In  reality,  this  book  is  based  upon  the  manuscript  of  another 
and  more  advanced  work,  which  is  soon  to  be  published  as 


PREFACE.  ix 

a  handbook  for  teachers  and  for  reference.  Both  of  these 
represent  an  attempt  to  gather  from  all  available  sources,  the 
kind  of  matter  which  it  seemed  desirable  to  include  in  such 
books.  While  in  the  larger  work  direct  reference  is  made 
to  the  sources  of  information,  it  has  not  seemed  desirable  to 
do  so  in  this  case ;  for  the  acknowledgments  take  much  space 
and  distract  the  attention,  without  benefiting  the  pupil. 

I  have  had  much  generous  assistance  in  the  supply  of  illus- 
trations, particularly  of  photographs ;  and  grateful  general 
acknowledgment  is  made  here,  while  special  mention  of  the 
sources  is  made  in  a  list  in  the  succeeding  pages.  Although 
I  have  received  aid  from  many  sources,  there  are  a  few  which 
I  must  mention  especially.  The  writings  of  Geikie,  Button, 
Powell,  and  Gilbert,  particularly  the  latter,  have  not  only 
given  me  bodies  of  fact,  but  also  inspiration,  as  indeed  they 
have  to  all  who  are  working  in  physiographic  geology.  To 
the  writings  and  teachings  of  Professors  Shaler  and  Davis 
of  Harvard  University,  I  owe  more  than  I  could  possibly 
acknowledge  ;  and  to  the  latter  I  am  under  an  added  obliga- 
tion for  his  examination  and  kindly  criticism  of  parts  of  my 
manuscript.  While  I  acknowledge  the  debt  which  I  owe 
these  scientists,  it  must  be  understood  that  the  mode  of 
presentation  is  my  own,  and  that  I  alone  am  responsible  for 
any  shortcomings  which  may  appear. 

RALPH  S.  TARR. 

Ithaca,  N.Y.,  August  30, 1895. 


CONTENTS. 

Part  I.     The  Air. 
CHAPTER  I.     The  Earth  as  a  Planet. 

PAOB 

Form  of  the  Earth 3 

The  Solar  System 5 

The  Sun 6 

The  Planets 8 

Asteroids 11 

The  Earth 11 

The  Moon * 13 

Comets,  Shooting  Stars,  and  Meteors 16 

The  Stellar  System 17 

Symmetry  of  the  Solar  System 18 

The  Nebular  Hypothesis 19 

Verification  of  the  Nebular  Hypothesis 20 

CHAPTER  II.    The  Atmosphere. 

General  Statement 23 

Light 25 

Electricity  and  Magnetism 29 

Heat 30 

Moisture 36 

Pressure 39 

Effect  of  Gravity 39 

Effect  of  the  Earth's  Rotation 39 

CHAPTER   III.     Distribution  of  Temperature. 

General  Statement ^ 

Effect  of  Atmospheric  Movements .44 

xi 


xn  CONTENTS, 

FAOB 

Influence  of  Oceans 46 

Effect  of  Topography      .         .        ,        , 47 

Seasonal  Temperature  Range .48 

Isothermal  Charts 51 

Daily  Temperature  Curve ,60 

Temperature  Ranges .62 

Earth  Temperatures        .        .        . .66 


CHAPTER  IV.     General  Circulation  of  the  Atmosphere. 

General  Statement 68 

Classification  of  the  Winds "*      .        .        .         .70 

Planetary  or  Permanent  Winds 71 

Trade  Winds .71 

Doldrum  Belt 74 

Anti-trade  Winds 74 

Horse  Latitude  Winds 75 

Prevailing  Westerlies 75 

Periodical  AVinds 76 

Seasonal  Winds .        .        .76 

Migrating  Wind  and  Calm  Belts 76 

Monsoon  Winds 77 

Diurnal  Winds 79 

Sea  and  Land  Breezes 79 

Mountain  and  Valley  Breezes   .        .        .        .        .        .        .80 

Eclipse  and  Tidal  Breezes 82 

Irregular  Winds 82 

Accidental  Winds 82 

The  Nature  of  Winds      ...;.;....      83 


CHAPTER  V.    Storms. 

Cyclonic  Storms      . 85 

Hurricanes 86 

Description        . 86 

Effects 88 

Path 90 

Time  of  Occurrence 91 

Cause 91 


CONTENTS.  xiii 


PAGE 


Temperate  Latitude  Cyclones 93 

Resemblance  to  Hurricanes 93 

Differences  from  Hurricanes ..06 

Effects       .        .        .        . 98 

Winds 98 

Anticyclones .        .        .  100 

Cause 100 

Secondary  Storms 101 

Thunderstorms         .         .         . 101 

Tornadoes  and  Waterspouts    . 104 


CHAPTER  VI.    The  Moisture  of  the  Atmosphere. 

Dew 107 

Frost 108 

Fog 109 

Haze .        .110 

Mist Ill 

Clouds Ill 

Rain .114 

Snow 115 

Hail 116 

Distribution  of  Rainfall  in  the  World 117 

Distribution  of  Rainfall  in  the  United  States 118 

Distribution  of  Snowfall          . 121 

Seasonal  Distribution  of  Rainfall 122 

Irregularities  of  Rainfall 123 


CHAPTER   VII.     Weather  and  Climate. 

Weather 124 

Tropical  and  Arctic 124 

Temperate  Latitude  Weather 126 

Climate 129 

Tropical  Climate 130 

Temperate  Climate l*^^ 

Arctic  Climate  .        .        . 132 

Minor  Variations •         •  1*^2 

Changes  in  Climate •  1^2 


xiv  CONTENTS. 


CHAPTER  VIII.     Geographic  Distribution  of  Animals 

AND  Plants. 

PAGE 

Geperal  Statement 135 

The  Ocean .        .        .        .        .135 

Eresh  Water 137 

The  Land        .        .        .        . 137 

Effect  of  Temperature  and  Moisture        ......     137 

Plant  and  Animal  Habits 141 

Life  Zones 143 

The  Spread  of  Life .145 

Barriers  to  the  Spread  of  Life 147 

Effect  of  Man 147 


Part  II.     The  Ocean. 

CHAPTER   IX.     Form  and  General  Characteristics  op 

THE  Ocean. 

Distribution  of  Land  and  Water     . 151 

Composition  of  Ocean  Water 151 

Color  and  Phosphorescence 152 

Exploration  of  the  Ocean  Bottom 153 

Methods  used  in  Deep-sea  Explorations 153 

Sounding 153 

Dredging 155 

Topography  of  the  Ocean  Bottom 156 

General 156 

The  Atlantic  Ocean 158 

Other  Oceans 160 

Topography  near  the  Coast      . 160 

Temperature  of  the  Ocean  Bottom          . 162 

Light  on  the  Ocean  Bottom 163 

Materials  composing  the  Ocean  Floor 164 

Mechanical  Sediments 164 

Globigerina  Ooze 164 

Red  Clay 165 

Life  in  the  Ocean 166 

Pelagic  or  Surface  Faunas        . 166 

Littoral  or  Shore  Faunas 167 

Faunas  of  the  Ocean  Bottom 169 


CONTENTS.  XV 
CHAPTER    X.    Ocean  Waves  and  Currents. 

PACK 

Wind  Waves .  174 

Earthquake  Waves 178 

Storm  Waves 179 

Ocean  Surface  Temperatures .        .        .  179 

Ocean  Currents       . 182 

Planetary  Circulation 182 

The  System  of  Ocean  Currents 183 

Cause  of  Ocean  Currents 185 

The  Gulf  Stream 187 

The  Labrador  Current      .        .        . 189 

Efiects  of  Ocean  Currents 189 


CHAPTER   XI.     Tides. 

Nature  of  the  Tidal  Wave .        .        .        .192 

Cause  of  Tides 192 

Effect  of  the  Land 193 

Other  Causes  for  Variation  in  Tidal  Height 198 

Effects  of  Tides 201 


Part  III.     The  Land. 
CHAPTER  XII.    The  Crust  of  the  Earth. 

Interior  Condition 205 

Movements  of  the  Crust 206 

Disturbance  of  the  Rocks 207 

Volcanic  Action 211 

Rocks  of  the  Earth's  Crust 212 

91 '-t 

Igneous  Rocks •**" 

Metamorphic  Rocks •        •        •        .214 

Sedimentary  Rocks 214 

Deposition  of  Sedimentary  Rocks 215 

Consolidation  of  Sedimentary  Rocks 217 

218 
Geological  Chronology 

Age  of  the  Earth ^^^ 


XVI  CONTENTS, 


CHAPTER   XIII.      Denudation  of  the  Land. 

PAGE 

Underground  Water .        .        .        .  224 

The  Formation  of  Caverns 226 

Springs  and  Artesian  Wells .  228 

Durability  of  Rocks 231 

Weathering 233 

Agents  of  Erosion 238 

Wind  Erosion 238 

Rain  Erosion 239 

Percolating  Water 240 

River  Erosion 241 

Ocean  Erosion 244 

Glacial  Erosion 245 

Denudation 246 


CHAPTER   XIV.     Topographic  Features  of  the  Earth's 

Surface. 

Continents  and  Ocean  Basins 249 

Physical  Geography  of  the  United  States 253 

Atlantic  Coast  Area "      .         .        .        .  254 

The  Eastern  Mountains 254 

The  Canadian  Highlands 256 

The  Central  Plains 256 

The  Cordilleran  Area 257 

The  Drainage  of  the  Country 259 

The  Shore  Line 261 


CHAPTER   XV.     River  Valleys. 

General  Description        .        .       " .  262 

Development  of  River  Valleys 265 

Adjustment  of  Streams 272 

The  River  Divide 273 

Accidents  to  Streams 275 

Land  Movements 276 

Climatic  Accidents 279 

Other  Accidents 282 


CONTENTS.  xvii 


CHAPTER   XVI.     Deltas,  Floodplains,  Waterfalls, 

AND  Lakes. 

PAOK 

Deltas 286 

Floodplains 288 

Waterfalls 294 

Lakes 298 

Swamps 303 

CHAPTER   XVIL     Glaciers. 

Cause  of  Glaciers    .        .        .        .        .        .        .        .        .        .        .  306 

Alpine  or  Valley  Glacier 307 

Continental  Glaciers 313 

Icebergs .        .        .        .  315 

Glacial  Period 316 

Area  covered  by  Ice 316 

Terminal  Moraine 319 

Formation  of  Soil 321 

Formation  of  Lakes 323 

Formation  of  Waterfalls 326 

CHAPTER   XVIII.     The  Coast  Line. 

General  Statement 328 

Effect  of  Elevation 329 

Effect  of  Depression 329 

Effect  of  Sediment 330 

Effect  of  Waves  and  Currents 332 

Effect  of  Plants 337 

Effect  of  Animals 340 

Changes  in  Coast  Form 343 

Islands 344 

Promontories 346 

Lake  Shores 347 

Fossil  Shore  Lines 348 

CHAPTER   XIX.     Plateaus  and  Mountains. 

Plateaus 360 

Mountains 353 

Characteristics  of  Mountains 363 


xviii  CONTENTS. 

FAGB 

The  Origin  of  Mountains 362 

Sculpturing  of  Mountains .  364 

The  Drainage  of  Mountains      .        .        .         .         .        .         .        .  365 

Destruction  of  Mountains 367 

CHAPTER  XX.     Volcanoes,  Earthquakes,  and  Geysers. 

Volcanoes *     .        .        .  370 

Distribution 370 

Materials  Erupted 371 

Eruptions  of  Volcanoes    .        .        .         .        .         .         .        .         .  374 

Form  of  Cone 378 

Effects  of  Volcanic  Eruptions 381 

Extinct  Volcanoes 381 

Cause  of  Volcanoes 383 

Earthquakes 383 

Geysers  and  Hot  Springs 386 

CHAPTER  XXI.     The  Topography  of  the  Land. 

General  Statement 390 

Constructive  Land  Forms .        .  392 

By  Internal  Forces  .        .        .        ., 392 

By  Agents  of  Denudation 393 

By  Animal  and  Plant  Life 395 

Effect  of  Rock  Structure  upon  Topography 395 

CHAPTER   XXII.    Man  and  Nature. 

General  Statement 407 

Modifying  Influence  of  Man 407 

Man  and  the  Forest 409 

Influence  of  Nature  upon  Man 412 

CHAPTER   XXIII.    Economic  Products  of  the  Earth. 

Soil 420 

Building  Stones 420 

Economic  Deposits  of  Sedimentary  Origin 422 

Miscellaneous  Substances 423 


CONTENTS.  xix 

PAGE 

Coal 423 

Natural  Gas  and  Petroleum     .        . 425 

Ore  Deposits 426 

Distribution  of  Ore  Deposits  .         . 428 

Mineral  Wealth  of  the  United  States 429 

APPENDIX  I. 

Meteorological  Instruments,  Apparatus,  and  Methods. 

Thermometric  Records 431 

Barometric  Records 432 

Measurement  of  Wind  Direction  and  Force 433 

Measurement  of  Evaporation 434 

Measurement  of  Moisture  in  the  Air 434 

Study  of  Clouds  and  Sunshine 434 

Measurement  of  Rainfall 435 

Meteorological  Methods  and  Results 435 

APPENDIX  II. 

Topographic  Maps 437 

APPENDIX  III. 

Suggestions  to  Teachers 440 

APPENDIX  IV. 

Questions  upon  the  Text    .        .  ' 453 


ILLUSTRATIONS. 


DIAGRAMS  AND   PHOTOGRAPHS. 

FIG.  PAGE 

1.  Sphere  and  oblate  spheroid 3 

2.  Land  and  water  hemispheres 4 

3.  The  solar  system 5 

4.  Relative  size  of  sun  and  large  planets 7 

5.  Sun  spots,  1872 8 

6.  Relative  distances  of  planets  from  the  sun        .        .        .        .        .  8 

7.  Relative  size  of  smaller  planets 9 

8.  Illustration  of  the  cause  of  seasons 12 

9.  Relative  size  of  earth  and  moon .  14 

10.  Lunar  craters 14 

11.  Comet  of  Donati,  1858 15 

12.  Orbit  of  comet  of  1862 16 

13.  Andromeda  nebula 17 

14.  Thickness  of  the  atmosphere 23 

15.  Decrease  in  density  of  the  atmosphere 24 

16.  Passage  of  sun's  rays  through  the  atmosphere          ....  26 

17.  Inclination  of  the  sun's  rays 34 

18.  Daily  change  in  relative  humidity 37 

19.  Increase  in  temperature  of  descending  air 38 

20.  Deflection  of  air  currents 40 

21.  Decrease  in  diameter  on  different  latitudes 40 

22.  Daily  temperature  curves 44 

23.  Irregularities  of  seasonal  curve 45 

24.  Seasonal  temperature  ranges 49 

25.  Seasonal  temperature  range  (New  York) 51 

26.  Isotherms  for  February  (northern  hemisphere)        ....  62 

27.  Daily  temperature  curve  (summer  and  winter)         ....  60 

28.  Daily  temperature  range  for  several  days          .        .        .        .        .  61 

29.  Daily  temperature  record  for  several  days 61 

xxi 


xxii  ILL  US  TEA  TIONS. 


riG 


PAGE 

30.  Temperature  ranges,  United  States,  1892 62 

31.  Minimum  temperatures,  United  States,  1892    .        .        .        .        .63 

32.  Maximum  temperatures,  United  States,  1892 64 

33.  Daily  temperature  range  near  and  above  the  ground       ...  66 

34.  General  circulation  of  the  globe 69 

35.  Summer  monsoons,  India 77 

36.  Winter  monsoons,  India 77 

37.  The  sea  breeze 78 

38.  The  land  breeze 79 

39.  Effect  of  sea  breeze  on  air  temperature 80 

40.  Valley  breeze 81 

41.  Ideal  diagram  of  a  storm 85 

42.  Barometric  record  during  passage  of  a  hurricane     ....  86 

43.  Diagram  of  hurricane  winds .87 

44.  Map  of  a  hurricane 88 

45.  Tracks  of  August  hurricanes 89 

46.  Map  of  temperate  latitude  cyclone 94 

47.  Paths  of  low-pressure  areas 95 

48.  Average  storm  tracks,  1878-1887  (Northern  hemisphere)        .        .  96 

49.  Tracks  of  low-pressure  areas 97 

60.  Photograph  of  thunderstorm 102 

51.  Path  of  thunderstorm .  103 

62.  View  of  a  tornado 104 

63.  Effect  of  tornado  at  Lawrence,  Mass 105 

64.  Distribution  of  tornadoes  (1794-1881) 106 

66.  Valley  fog  in  the  Himalayas      .        . 110 

66.  The  banner  cloud Ill 

67.  Photographs  of  clouds 112 

68.  Photographs  of  snowflakes 115 

69.  Damp  snowfall 116 

60.  Evaporation  in  United  States 120 

61.  Monthly  rainfall  in  the  West 121 

62.  Variation  in  annual  rainfall  in  the  West 122 

63.  A  cold  wave 127 

64.  Temperature  descent  during  cold  wave 128 

66.  Climatic  zones 129 

66.  Near  the  timber  line 138 

67.  Above  the  snow  line,  Mount  St.  Elias,  Alaska         ....  139 

68.  Effect  of  sunlight  on  mountain  vegetation 140 

69.  Arid  land  vegetation 141 

70.  Arid  land  vegetation,  Rio  Grande  valley 142 


ILLUSTRATIONS,  xxiii 

FIG.  PAGE 

71.  The  tropical  forest 143 

72.  Life  zones  of  United  States 144 

73.  Deep-sea  sounding  machine 154 

74.  Deep-sea  trawl 155 

75.  Contrast  between  land  and  ocean  bottom  topography    .        .        .  156 

76.  Cross-section  of  Atlantic  Ocean 158 

77.  Temperature  of  the  Mediterranean          .        .        .        .        .        .  163 

78.  Globigerina  ooze 165 

79.  Coral  reef  on  Australian  coast 168 

80.  Ocean  waves 174 

81.  Breakers  on  the  coast 175 

82.  Effect  of  storm  waves  on  the  coast 177 

83.  Normal  vertical  descent  of  ocean  temperatures      ....  180 

84.  Tides  near  Hell  Gate,  N.Y 196 

85.  Time  and  height  of  tides  at  Hell  Gate 196 

86.  The  tides  at  Eastport,  Me.,  September,  1893 199 

87.  Height  of  high  tide,  Eastport,  Me.,  1893  and  1894  .        .        .200 

88.  Tidal  mud  flat  in  Bay  of  Fundy 202 

89.  Tidal  rise  and  fall.  Cape  Ann,  Mass.       ......  203 

90.  Horizontal  rocks  in  Kansas 208 

91.  A  monoclinal  fold 208 

92.  Anticline 208 

93.  Syncline 209 

94.  Photograph  of  anticline,  Hancock,  W.Va 209 

95.  Photograph  of  anticline  near  Quebec,  Canada        ....  209 

96.  Photograph  of  a  fault  in  Arizona 210 

97.  Photograph  of  a  fault  in  glacial  clay,  Massachusetts      .        .        .  210 

98.  A  dike  crossing  granite 212 

99.  Contorted  limestone 214 

100.  Stratified  shale,  near  Ithaca,  N.Y 215 

101.  Section  of  alternating  strata     . 216 

102.  Unconformity  in  horizontal  rocks 217 

103.  Unconformity  in  inclined  rocks 217 

104.  Photograph  of  fossiliferous  rock 219 

105.  Mammoth  Hot  Springs,  Yellowstone  Park 225 

106.  Diagram  illustrating  formation  of  caverns 226 

107.  A  sink  hole  in  limestone  region 227 

108-  Stalactites  in  Luray  Cave 227 

109.  The  Natural  Bridge,  Va 228 

110.  A  spring  on  a  fault  plane         .        . 228 

111.  Hillside  spring 229 


XXIV 


ILLUSTRATIONS. 


FIG. 

112.  Photograph  of  an  artesian  well 

113.  Artesian  well  in  monoclinal  strata  . 

114.  Artesian  well  in  syncline 

115.  Rock  pillars  in  Garden  of  Gods,  Col 

116.  The  weathering  of  granite 

117.  Effect  of  roots  in  breaking  up  rocks 

118.  Talus  in  Rio  Grande  valley,  N.M. 

119.  The  formation  of  residual  soil 

120.  Sand  dunes,  Cape  Ann,  Mass. 

121.  Moqui  pueblo,  New  Mexico      . 

122.  Talus  furnishing  load  to  river  . 

123.  Yellowstone  valley,  broadening  by  weathering 

124.  Boulder  bed  of  Westfield  River,  Mass. 

125.  Sea  cliffs  on  volcanic  island     . 

126.  Granite  hill  rounded  by  glacial  action 

127.  Relief  map  of  Eurasia 

128.  Section  across  the  Atlantic  and  United  States 

129.  Relief  map  of  North  America  . 

130.  A  deep  mountain  valley  .... 

131.  Stream  issuing  from  a  limestone  cave 

132.  Brink  of  Niagara  Falls     .... 

133.  Gorge  near  Ithaca,  N.Y 

134.  Royal  Gorge,  Col 

135.  Oxbow  cut-off  in  Connecticut  valley 

136.  Development  of  the  canon 

137.  Development  of  the  caiion  profile    . 

138.  Development  of  old  valley 

139.  The  Yellowstone,  broadening  by  weathering 

140.  A  bit  of  Illinois  drainage 

141.  A  bit  of  West  Virginia  drainage 

142.  Cafion  of  the  Colorado     .... 

143.  A  broad  Alpine  valley      .... 

144.  Mountain  gorge  in  the  Alps     . 

145.  Diagram  illustrating  change  in  divide 

146.  Diagram  illustrating  change  in  divide     . 

147.  Diagram  illustrating  monoclinal  shifting 

148.  Diagram  illustrating  sudden  change  in  divide 

149.  Effect  of  elevation  on  Colorado  cafion     . 

150.  The  drainage  of  an  arid  region 

151.  The  Great  Basin 

152.  Effect  of  glaciation  on  stream  courses     . 


PAGE 

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230 
230 
231 
233 
235 
236 
237 
238 
239 
240 
242 
243 
244 
245 
250 
251 
252 
262 
263 
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265 
265 
266 
267 
267 
267 
268 
269 
269 
270 
271 
272 
273 
274 
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275 
276 
280 
281 
282 


ILLUSTBATIONS,  xxv 


FIO. 


PAGE 


153.  Delta  of  the  Mississippi 286 

154.  Mode  of  formation  of  a  delta  . 288 

155.  An  alluvial  fan 288 

156.  Floodplain  among  mountains 289 

157.  Floodplain  of  a  great  river 290 

158.  Meandering  of  the  Mississippi .  291 

159.  Meandering  of  the  Mississippi 292 

160.  Meandering  of  the  Mississippi          .        .        .        .        .        .        .  292 

161.  Falls  of  the  Yellowstone  .        .        .        .        .        .        .        .        .  293 

162.  Taughannock  Falls,  N.Y .        .294 

163.  American  Falls,  Niagara .        . 295 

164.  Yosemite  Falls 296 

165.  Falls  in  a  gorge  near  Ithaca,  N.Y .        .  297 

166.  Diagram  illustrating  origin  of  Niagara 298 

167.  River  valley  transformed  to  a  lake  (Adirondacks)         ...  299 

168.  Glacial  lakes  in  the  Adirondacks     .......  300 

169.  Bird's-eye  view  of  Niagara  River 301 

170.  Shore  lines  of  extinct  Lake  Bonneville 302 

171.  A  Florida  swamp 303 

172.  Ray  Brook,  Adirondacks .        .  304 

173.  An  Alpine  snow  field .        .  306 

174.  Whitney  Glacier,  Mount  Shasta 307 

175.  The  Rhone  glacier 308 

176.  Crevasse  in  a  glacier .        .        .  309 

177.  Glacier,  Mount  Dana,  Cal.       .        . 310 

178.  Section  of  a  glacier 312 

179.  Ice  cave  at  terminus  of  a  glacier 312 

180.  Forest  at  foot  of  Malaspina  Glacier,  Alaska  .        .        .        .        .  313 

181.  A  Nunatak  in  Greenland 314 

182.  Icebergs  in  the  Antarctic 315 

183.  An  iceberg  in  water 316 

184.  Glacial  lakes  and  moraine  in  a  mountain  valley     ....  31-7 

185.  Extent  of  the  continental  ice  sheet  in  America       ....  318 

186.  Boulder  in  moraine,  Cape  Ann,  Mass.     ......  320 

187.  Bear-den  moraine,  Cape  Ann,  Mass 321 

188.  Boulder-strewn  till  soil  in  Maine '      .  321 

189.  Glacial  scratches  on  a  pebble 322 

190.  Glacial  lakes  in  Massachusetts 324 

191.  Watkins  Glen,  N.Y .        .326 

192.  Sea  cliff,  Cape  Cod,  Mass 328 

193.  Submerged  valley  on  the  coast  of  Mount  Desert,  Me.    .        .        .  330 


XXVI 


ILL  US  TEA  TI0N8. 


FIO. 

194.  Ocean  bar  on  the  Texas  coast 

195.  Destruction  of  Heligoland  by  the  ocean 

196.  Lake  Spit 

197.  Hook,  Lake  Michigan   .... 

198.  Sea  cave  in  granite  rock,  Cape  Ann,  Mass. 

199.  Effect  of  dike  on  form  of  coast.  Cape  Ann,  Mass. 

200.  Pond  formed  by  beach  barrier.  Cape  Ann,  Mass. 

201.  Crescent-shaped  beach.  Cape  Ann,  Mass. 

202.  Boulders  worn  from  headland  by  waves 

203.  Rocky  beach  on  exposed  coast.  Cape  Ann,  Mass. 

204.  Mat  of  seaweed  between  tides.  Cape  Ann,  Mass. 

205.  A  mangrove  swamp       .... 

206.  Salt  marsh.  Cape  Ann,  Mass. 

207.  Coral  reef  on  the  Australian  coast 

208.  Keys  on  the  Florida  coast 

209.  An  atoll  in  the  Pacific   .... 

210.  Diagram  illustrating  origin  of  atolls 

211.  The  coast  of  Casco  Bay,  Me. 

212.  Cliff  on  the  shore  of  Lake  Michigan 

213.  Lagoon  enclosed  behind  lake  beach 

214.  Plain  in  Pecos  Valley,  N.M.  . 

215.  Plain  in  valley  of  Red  River  of  the  North 

216.  Taos  Mountains,  N.M 

217.  Plateau  near  Colorado  River 

218.  Butte  in  New  Mexico    .... 

219.  Talus  slope  in  the  Elk  Mountains,  Col. 

220.  Matterhorn,  Switzerland 

221.  A  mountain  park  (Baker's)  , 

222.  Mountain  gorge  in  the  Peruvian  Andes 

223.  Mount  of  the  Holy  Cross,  Col. 

224.  Trail  on  Long's  Peak,  Col.     . 

225.  Mountain  ridge  on  the  Canadian  Pacific 

226.  Section  across  a  mountain  ridge    . 

227.  A  bit  of  mountain  drainage  . 

228.  Map  of  mountain  drainage    . 

229.  Diagram  illustrating  the  development  of  a  mountain 

230.  A  mountain  ridge  in  Colorado 

231.  Vesuvius  in  eruption,  1872    . 

232.  Surface  of  a  recent  lava  flow 

233.  Lake  formed  by  a  lava  dam  . 

234.  Volcano  in  the  Lipari  Islands 


PAGE 

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332 
333 
333 
334 
335 
335 
336 
336 
337 
338 
338 
339 
341 
341 
342 
343 
345 
347 
348 
350 
350 
351 
352 
353 
354 
355 
357 
358 
359 
360 
361 
364 
365 
366 
367 
368 
372 
373 
374 
375 


ILLUSTRATIONS, 


XXVU 


FIG. 

235.  Disruption  of  Krakatoa         .... 

236.  Vesuvius,  from  Pompeii        .         .         . 

237.  Mount  Hood  —  an  apparently  extinct  volcano 

238.  Muir's  Butte,  Cal.,  —  a  recent  volcano 

239.  Fusiyama,  a  Japanese  volcano 

240.  Angle  of  slope  of  volcanic  cones    . 

241.  Mounts  Shasta  and  Shastina 

242.  Mato  Tepee,  Wyo., — a  volcanic  neck  .     .    . 

243.  Diagram  illustrating  the  earthquake  wave    . 

244.  Waves  of  Charleston  earthquake  .        .        . 

245.  Earthquake  shock  in  Japan  .... 

246.  Effect  of  earthquake  in  Japan,  1891 

247.  Fault  line  associated  with  Japanese  earthquake  of  1891 

248.  Crater  of  Oblong  Geyser,  Yellowstone  Park 

249.  Old  Faithful  Geyser,  Yellowstone  Park 

250.  Etching  of  hard  layer  by  denudation,  Brazil 

251.  A  cliff  in  the  Yosemite  .... 

252.  Cliffs  in  the  loess  of  China     . 

253.  A  wave-worn  chasm,  Gloucester,  Mass. 

254.  A  rugged  granite  coast.  Cape  Ann,  Mass. 

255.  A  sloping  granite  coast.  Cape  Ann,  Mass. 

256.  Effect  of  hard  layers  on  topography 

257.  Signal  Butte,  Tex 

258.  Effect  of  tilted  layers  on  topography     . 

259.  Form  of  seacoast  in  inclined  strata 

260.  Form  of  seacoast  in  inclined  strata 

261.  Ridge  of  hard  rock,  etched  by  denudation 

262.  Topography  in  region  of  folded  rocks    . 

263.  A  part  of  the  Adirondack  forest    . 

264.  Deforesting  of  the  Adirondacks     . 

265.  Bare  rock  exposed  by  removal  of  forest 

266.  Model  of  Cumberland  Valley,  Penn.    . 

267.  Hachure  map 


PAGE 

375 
376 
378 
379 
380 
380 
382 
383 
384 
384 
385 
386 
387 
388 
389 
396 
398 
399 
400 
401 
401 
402 
402 
403 
403 
403 
404 
405 
409 
410 
411 
437 
438 


xxviu 


ILLUSTRATIONS. 


PLATES   AND   CHARTS. 


PLATK 

1.  Watkins  Glen,  New  York 

2.  Isotherms  for  the  year  (world) 

3.  Isotherms  for  the  year  1892  (United  States) 

4.  Isothermal  chart  for  July  (world)  . 

5.  Isothermal  chart  for  January  (world)     . 

6.  Isothermal  chart  for  July  (United  States) 

7.  Isothermal  chart  for  January  (United  States) 

8.  Isothermal  chart  of  New  York  (year)     . 

9.  Winds  and  isobars  for  January  (world) 

10.  General  circulation  of  the  Atlantic,  July 

11.  General  circulation  of  the  Atlantic,  January 

12.  Rainfall  chart  of  the  world 

13.  Rainfall  chart  oi  the  United  States 

14.  Depths  of  the  ocean        .... 

15.  Ocean  surface  temperature,  Atlantic 

16.  Oceanic  circulation  .... 

17.  Gulf  Stream 

18.  Co-tidal  lines 

19.  English  Channel  tides     .... 

20.  Earth  columns.  New  Mexico  . 

21.  The  Bad  Lands  of  South  Dakota    . 

22.  Relief  map  of  the  United  States 

23.  Drainage  areas  of  the  United  States 

24.  Delaware  and  Chesapeake  bays 
26.  Drainage  in  glaciated  region,  Wisconsin 

26.  White  Glacier,  Alaska    .... 

27.  Distribution    of    volcanoes    and    ocean    surface 

(world) 

28.  Marble  Caflon,  Colorado  River 

29.  Navajo  church,  Arizona  .... 


facing 
facing 


facing 


PAGE 

Frontispiece 
facing     50 
.      54 
55 
56 
57 
58 
59 
70 
72 
73 
facing  117 
119 
161 
181 
facing  183 
188 
facing  194 
195 
232 
247 
facing  253 
260 
277 
283 
311 
temperatures 

facing  370 
.  391 
.     397 


ILL  USTRATIONS,  xxix 


ACKNOWLEDGMENT  OF  ILLUSTRATIONS. 

The  following  illustrations  are  from  the  sources  indicated.  In  some 
cases  they  have  been  exactly  reproduced,  but  in  others  they  have  been  made 
more  diagrammatic  to  suit  the  needs  of  this  book.  Some  of  the  illustrations 
not  acknovv^ledged  are  from  photographs  or  lantern  slides,  the  source  of 
w^hich  could  not  be  ascertained,  i 

Abbe,  U.  S.  S.  S.,  Annual  Report  for  1890,  Fig.  56. 

Agassiz,  Three  Cruises  of  the  Blake,  Plate  15. 

Branner,  Journal  of  Geology,  Vol.  1,  Fig.  250. 

Brown,  C.  D.  (dealer  in  photographs,  Gloucester,  Mass.),  Figs.  81,  89,  203, 

204,  and  254. 
Buchan,  Atmospheric  Circulation,  Challenger  Reports,  Plates  2,  4,  5,  and  9. 
Calvin,  Prof.  S.,  State  Geologist  of  Iowa,  Des  Moines, — Photograph  by  the 

Survey,  Fig.  131. 
Canadian  Geological  Survey,  Photograph,  Fig.  99. 
Challenger  Reports,  Narrative,  Figs.  78,  125,  and  182. 
Chamberlin,  Third  Annual  Report,  U.  S.  G.  S.,  Fig.  185. 
Diller,  Bulletin  79,  U.  S.  G.  S.,2  Figs.  232,  233. 
Dunwoody,  Summary  of  International  Meteorological  Observations,  Figs.  26 

and  48 ;  same.  Professional  Paper  IX.,  U.  S.  S.  S.,  Plate  13. 
Button,  Second  Annual  Report,  U.  S.  G.  S.,Figs.  137  and  149;  same.  Sixth 

Annual,  Plate  29  ;  same.  Ninth  Annual,  Fig.  244;  same,  Monograph  II., 

U.  S.  G.  S.,  Fig.  136. 
Ferrel,  Popular  Treatise  on  the  Winds,  Fig.  34. 
Finley,  U.  S.  S.  S.,  Professional  Paper  VII.,  Fig.  54. 
Gannett,  Thirteenth  Annual  Report  U.  S.  G.  S.,  Plate  22. 
Gardner,  J.  L.,  2d,  Boston,  Mass.  (Photographs  by).  Figs.  98,  116,3  117,3 

120,  126,3  186,3  187,  198,3  199,3  20O,  201,  202,3  206,  and  255.3 
Gilbert,  Monograph  I.,  U.  S.  G.  S.,  Figs.  151,  154,  155,  170,  197,  and  213  ; 

same.  Fifth  Annual  Report  U.  S.  G.  S.,  Figs.  212  and  217  ;  same.  Annual 

Report  Smithsonian  Institution,  1890,  Figs,  166  and  169  ;  same.  Geology 

of  the  Henry  Mountains,  Fig.  147. 

1  U,  S.  C.  S.,  refers  to  the  United  States  Coast  Survey  ;  U,  S,  G,  8.,  to  the  United  States 
Geological  Survey ;  and  U.  S.  S,  S.,  to  the  United  States  Signal  Service. 

2  Some  of  these  which  are  referred  to  the  Geological  Survey  publication  were  made  from 
photographs  obtained  from  the  Survey. 

'  Also  published  by  Shaler  in  Ninth  Annual  Report,  U,  S.  G.  S. 


XXX  ILLUSTRATIONS. 

Greely,  U.  S.  S.  S.,  Professional  Paper  II.,  Plates  6  and  7. 

Griswold,  L.  S.,  Dorchester,  Mass.  (Photograph  by).  Fig.  97. 

Guyot,  Physical  Geography,  Pig.  2. 

Ilann,  Berghaus,  Atlas  der  Meteorologie,  Plate  12. 

Hann,  Hochstetter,  and  Pokorny,  Allgemeine  Erdkunde,  Fig.  77. 

Harvard  College  Astronomical  Observatory  Engravings,  Figs.  5,  11,  and  13. 

Harvard  College  Geological  Department,  Figs.  215  and  231  (former,  photo- 
graph from  South  Dakota  World's  Fair  Commissioner ;  latter,  pho- 
tograph by  Sommer). 

Haynes,  F.  Jay,  St.  Paul,  Minn.  (Photographer),  Figs.  105,  123,  139,  161, 
248,  249. 

Hellmann,  Schneekrystalle,  Fig.  58. 

Hill,  First  Annual  Report,  Texas  Geological  Survey,  Fig.  257. 

Hope,  J.  D.,  Photographer,  Watkins,  N.Y.,  Plate  1  and  Fig.  191. 

Jackson  Photograph  Co.,  Denver,  Col.,  Figs.  134,  221,  224,  237,  238,  and  251. 

James,  C.  H.,  Photographer,  Philadelphia,  Pa.,  Fig.  108. 

Jukes-Browne,  Handbook  of  Physical  Geology,  Fig.  195. 

Kent,  Great  Barrier  Reef,  Figs.  79  and  207. 

Kobayashi,  Earthquake  Observations  in  Japan,  Fig.  245. 

Koppen,  Segelhandbuch  fiir  den  Atlantischen  Ozean  (reproduced  by  Davis, 
American  Meteorological  Journal,  Vol.  IX.),  Plates  10  and  11. 

Lesley,  Coal  and  its  Topography,  Figs.  256  and  262. 

Levy  and  Co.,  Paris  (Dealers  in  Photographs),  Figs.  143,  144,  175,  and  220. 

Merriam,  North  American  Fauna,  Bulletin  No.  3,  U.  S.  Dept.  of  Agriculture, 
Fig.  08 ;  same.  National  Geographic  Magazine,  Vol,  VI.,  1894,  Fig.  72. 

Mills,  H.  F.,  Annals,  Harvard  College  Astronomical  Observatory,  Vol.  31, 
Fig.  63. 

Mills,  H.  R.,  Realm  of  Nature,  Plates  16  and  27. 

Mississippi  River  Commission  (Maps),  Figs.  158,  159,  and  160. 

Mitchell,  U.  S.  C.  S.,  Annual  Report  for  1886,  Fig.  85. 

Murray  and  Renard,  Challenger  Reports  — Deep  Sea  Deposits,  Plate  14. 

Nasmyth  and  Carpenter,  The  Moon,  Fig.  10. 

Newconib,  Popular  Astronomy,  Fig.  12. 

Newell,  Eleventh  Census  Report  on  Irrigation,  Figs.  61  and  62. 

Newton  &  Co.,  London,  England  (Dealers  in  Lantern  Slides),  Figs.  52,  55, 
71,  106,  181,  205,  209,  234,  and  339. 

New  York  State  Weather  Bureau,  Fifth  Annual  Report,  Plate  8  and  Fig.  25  ; 
Figures  based  on  the  records  of  this  bureau  :  18,  28,  29,  33,  42,  and  64. 

Notman  (Photographer),  Montreal,  Canada,  Fig.  225. 

Pach  (Photographer),  New  York,  N.Y.,  Fig.  82. 

Peschels  (Leipoldt),  Physische  P^rdkunde,  Plates  18  and  19. 

Pillsbury,  Annual  Report,  U.  S.  C.  S.  for  1890,  Plate  17. 


ILLUSTRATIONS.  XXxi 

Proctor  Bros.  (Dealers  in  Photographs),  Gloucester,  Mass.,  Pig.  80. 

Reid,  National  Geographic  Magazine,  Vol.  IV.,  Plate  26. 

Richthofen,  China,  Fig.  252. 

Riggenbach  (Photographs),  Figs.  50  and  57  (latter  from  several  sources) . 

Ritchie,  J.,  Jr.,  Boston,  Mass.  (Photographs  by).  Figs.  124  and  188. 

Russell,  Fifth  Annual  Report,  U.  S.  G.  S.,  Fig.  177  ;  same.  Eighth  Annual, 

Fig.  184 ;  same.  Thirteenth  Annual,  Figs.  67  and  180. 
Sella  (Photographs;  Chas.  Pollock,  Boston,  Agent),  Figs.  176  and  179. 
Shaler,  Twelfth  Annual  Report,  U.  S.  G.  S.,  Figs.  107,  157,  and  171. 
Sigsbee,  U.  S.  C.  S.,  Deep  Sea  Sounding  and  Dredging,  Figs.  73  and  74. 
Smith,  W.  M.  (Dealer  in  Photographs,  Provincetown,  Mass.),  Fig.  192. 
Stoddard,  S.  R.  (Photographer),  Glens  Falls,  N.Y.,  Figs.  88,  167,  168,  172, 

193,  263,  264,  and  265. 
Symons,  Eruption  of  Krakatoa,  Fig.  235. 
Todd,  Bulletin  I.,  South  Dakota  Geological  Survey,  Fig.  112. 
Trotter,  Lessons  in  the  New  Geography,  Figs.  127  and  129. 
United  States  Coast  Survey  Charts,  Figs.  153,  194,  208,  211,  267,  and  Plate  24. 
United  States  Geological  Survey  Photographs,  Figs,  m,  94,  95,  96,  119,  122, 

132,  142,  163,  174,  196,  230,  241,  242,  261,  and  Plate  28  ;  same,  Topo- 
graphic Maps,  Figs.  150,  190,  228,  and  Plate  25. 
United  States  Geological  Survey  of  the  Territories  (Hay den),  Photographs, 

Figs.  69,  115,  121,  130,  156,  164,  219,  223. 
United  States  Hydrographic  Bureau  (Coast  Pilot),  Figs.  43,  44,  45. 
United  States  Signal  Service  and  Weather  Bureau,  Figs.  30,  31,  32,  46,  47, 

49,  60,  63,  and  Plate  3. 
Van  Bebber,  Lehrbuch  der  Meteorologie,  Fig.  41 . 
Walcott,  National  Geographic  Magazine,  Vol.  V.,  Fig.  109. 
Ward,  Annals  Harvard  College  Astronomical  Observatory,  Vol.  31,  Fig.  51. 
Wild,  Thalassa,  Fig.  21. 

Willis,  Thirteenth  Annual  Report,  U.  S.  G.  S.,  Figs.  92,  93,  and  101. 
Williston,  Prof.  S.  W. ,  Kansas  University  Geological  Department,  Lawrence, 

Kansas  (Photograph  by).  Fig.  90  and  Plate  21. 


Part  I. 

THE   AIR. 

WITH  AN  INTRODUCTORY  CHAPTER  ON  THE  ASTRONOMICAL 

RELATIONS   OF   THE   EARTH. 


o  > 


ELEMENTAEY  PHYSICAL  GEOGRAPHY. 


-OKi'i^OO- 


CHAPTER   I. 


THE  EARTH  AS   A  PLANET. 


Form  of  the  Earth.  —  The  earth  is  a  spherical  body  com= 
posed  of  tliree  different  portions, — a  dense  central  mass,  which 
is  probably  solid,  and  two  envelopes,  the  ocean  and  the  air. 
The  central  part  has  a  much  greater  bulk  than  either  of  the 
other  portions.  In  reality  the  form  is  not  exactly  spherical, 
for  the  diameter  of  a  sphere  should  have  the  same  length  in 
all  parts ;  but  on  the  earth  the  diameter  at  the  equator  is  26^ 
miles  longer  than  that  at  the  poles, 
where  its  length  is  7899  miles. 
This  flattening  of  the  poles  gives 
to  the  earth  the  form  of  an  oblate 
spheroid  instead  of  a  true  sphere 

(Fig-  1)- 

While  this   irregularity  of  the 

earth  was  detected  only  after  a 
series  of  very  careful  measure- 
ments, it  is  in  reality  the  greatest 
on  the  surface  of  the  earth;  but 
there  are  other  and  less  extensive 
irregularities,  which  are  much 
more  noticeable.  These  are  of  two  kinds,  —  continents  and 
mountains.  The  surface  rises  and  falls  in  a  series  of  great 
wave-like  irregularities,  which  form  the  continents  and  ocean 

3 


Fig.  1. 

Diagram  showing  a  section  of 
a  sphere  (heavy  line) ,  and  an 
oblate  spheroid  (dotted  line). 


f  <^   <■  ^    >        «    iTt<         • 


*    "« 


PHYSICAL    GEOGBAPHY. 


basins.  On  the  continents,  and  occasionally  in  the  oceans, 
the  surface  rises  along  relatively  narrow  lines  into  a  series 
of  high  mountain  ridges.  Although  these  are  the  greatest 
elevations  on  the  earth's  surface,  and  therefore  attract  our 
attention,  they  are  really  very  small  irregularities  when  com- 
pared with  the  continents  of  which  they  usually  form  a  small 
portion  (Fig.  128). 

Considering  the  sea  level  as  0,  the  highest  point  on  the 
earth  is  about  29,000  feet  in  elevation.  Depressions  of  over 
25,000  feet  are  found  in  several  places  in  the  ocean  beds. 
The  total  range  in  elevation  between  the  highest  mountain, 
and  the  greatest  ocean  depth  is  about  57,000  feet.  It  can  be 
readily  seen  how  small  this  is  in  comparison  with  the  earth 
as  a  whole,  when  we  remember  that  the  diameter  of  the  earth 
at  the  equator  is  41,847,192  feet.  Upon  a  globe  of  ordinary 
size  they  could  not  be  shown  on  true  scale.  Although  there 
are  points  on  the  land  whose  height  is  greater  than  the 
deepest  known  parts  of  the  ocean,  the  average  depth  of 
the  ocean,  which  is  about  12,000  feet,  is  much  greater  than 
the  average  height  of  the  land,  which  is  approximately  2500 

feet  (see  Chap.  XIV.). 
The  greater  part  of  the 
water  on  the  earth's  sur- 
face is  accumulated  in  the 
broad  hollows  between 
the  continents.  The  sur- 
face of  this  water  mass  is 
much  greater  in  area  than 
that  of  the  land  (Fig.  2), 
the  proportion  being  1  of 
land  to  2. 6  of  water  (roughly  3:8).  Late  calculations  give  the 
area  of  the  land  as  142,000,000  square  kilometers,  and  of  the 
water  as  368,000,000  square  kilometers.     The  total  volume  of 


Fio.  2. 
Land  and  water  hemispheres. 


THE  EARTH  AS  A  PLANET.  6 

the  water  of  the  oceans  is  estimated  to  be  1,347,874,850  cubic 
kilometers. 

There  are  other  smaller  irregularities  on  the  surface  of  the 
earth,  and  many  minor  peculiarities,  some  of  which  are  dis- 
cussed in  the  later  chapters.  Surrounding  the  earth  is  a 
gaseous    envelope,   the    atmosphere,   which   extends   to   an 


JCS  years 
^'<'ptune 

Fig.  3. 

The  solar  system,  showing  the  relative  distances  from  the  sun,  the  direction  of 
revolutions,  relative  size  of  the  orbits,  and  the  number  of  satellites. 


unknown  distance,  but  which  at  a  height  of  five  or  six  miles 
from  the  surface  is  very  much  rarified. 

The  Solar  System.  —  The  earth  is  one  of  several  bodies 
which  together  form  the  solar  system.  They  are  a  family  of 
bodies  bound  together  by  the  tie  of  gravitation  and  engaged 
in  a  series  of  movements  around  a  central  body,  the  sun 
(Fig.   3).     In   the   solar  system   there   are   five   classes   of 


6  PHYSICAL    GEOGBAPHT. 

bodies.  In  the  center  is  the  sun,  the  largest  of  all,  and  the 
one  upon  which  the  others  depend  more  than  upon  any  other 
member.  The  second  class  of  bodies  is  that  of  the  planets, 
of  which  eight  are  known.  These  all  revolve  around  the 
sun  in  orbits  which  are  nearly  circular,  but  not  exactly  so, 
being  in  reality,  ellipses  with  the  sun  at  one  of  the  foci. 
The  third  class  of  bodies  is  that  of  the  satellites,  of  which 
the  moon  is  an  example.  Most  of  the  planets  have  satellites, 
which  are  always  much  smaller  than  the  planet  about  which 
they  revolve.  The  earth  has  but  one  moon,  but  some  of  the 
planets  have  several.  Twenty  moons  have  already  been 
discovered,  of  which  all  but  three  belong  to  the  outer  group 
of  planets,  Jupiter,  Saturn,  Uranus,  and  Neptune.  A  fourth 
group  of  bodies  in  the  solar  system  is  that  of  the  asteroids, 
of  which  about  400  are  now  known.  These  small  planets 
revolve  about  the  sun  in  the  space  between  the  orbits 
of  Mars  and  Jupiter.  Aside  from  these  members,  there  is  a 
fifth  group  of  irregular  bodies,  the  comets  and  meteors, 
which  move  in  a  manner  different  from  that  of  the  other 
members  of  the  solar  system. 

The  Sun.  —  The  central  and  largest  member  of  the  solar 
system,  the  sun  itself,  unlike  the  planets,  is  so  constituted 
that  it  sends  out  into  space  a  form  of  energy  which  produces 
both  light  and  heat.  It  is  the  source  of  much  of  the  energy 
which  finds  expression  upon  the  surface  of  the  earth  in  the 
forms  of  light,  heat,  and  life  itself.  This  immense  body  is 
fully  92,750,000  miles  distant  from  the  earth. 

Since  the  sun  is  able  to  emit  rays  which  produce  heat,  we 
know  that  it  must  be  a  hot  body  ;  but  there  is  as  yet  no 
means  of  telling  what  its  temperature  is.  Owing  to  the  way 
it  affects  the  movements  of  the  several  members  of  the  solar 
system,  it  is  known  that  the  materials  composing  the  sun  are 
not  so  dense  as  the  solid  part  of  the  earth.     It  seems  quite 


THE  EABTH  AS  A  PLANET. 


certain  that  at  least  a  large  part  of  the  sun  is  in  the  form 
of  gas.  By  means  of  the  instrument  known  as  the  spectro- 
scope^ we  have  learned  much  concerning  the  actual  composi- 
tion of  the  sun.  By  this  instrument  it  has  been  found  that 
many  of  the  elements  known  on  the  earth  exist  in  the  sun 
in  a  gaseous  form. 

Since  we  know  very  little  about  the  condition  of  the 
earth  on  which  we  live,  it  is  hardly  to  be  expected  that  our 
knowledge  of  a  body  so  distant  as  the  sun  would  be  very 
accurate.  Still  the  studies  which  have  been  carried  on  by 
means  of  the  telescope  have  revealed  the  fact  that  there  are  at 
least  three  quite  different 
parts  to  the  sun.  These 
are  the  corona,  which  is 
outermost,  the  chromo- 
sphere^ ?Lndit\iQ  photosphere^ 
the  latter  being  the  densest 
part.  It  is  the  portion 
from  which  the  light  and 
heat  are  emitted ;  and 
from  its  surface  the  diame- 
ter of  the  sun  is  about 
860,000  miles  (Fig.  4). 
Above  the  photosphere 
comes  the  chromosphere, 
which  appears  to  be  the 
true  atmosphere  of  the  sun.  It  consists  mainly  of  glowing 
hydrogen  gas  ;  but  in  its  lower  portions  many  metals,  such  as 
iron,  are  known  to  exist  in  the  form  of  gas.  It  is  in  violent 
commotion,  as  if  in  eruption  ;  and  the  photosphere  itself  also 
presents  signs  of  violent  activity.  Extending  to  a  distance 
sometimes  as  great  as  300,000  miles  above  the  surface  of  the 
sun,  is  the  corona,  the  character  of  which  is  not  understood. 


Fig.  4. 

Diagram  to  show  the  relative  size  of  the 
sun  and  the  largest  planets, 
true  scale. 


Drawn  on 


8  PHYSICAL    GEOGRAPHY. 

Certain  peculiar  spots  known  as  sun  spots  (Fig.  5)  appear 
upon  the  surface  of  the  sun  and  move  across  its  face  until 

they  disappear  on  the  opposite 
side,  being  carried  around  by  the 
rotation  of  the  sun.  Their  origin 
is  not  known,  but  they  appear  to 
have  an  influence  upon  the  earth 
in  at  least  two  ways,  one  upon 
atmospheric  electricity,  the  other 
upon  certain  climatic  features. 

The  sun  is  engaged  in  two  mo- 
tions. It  rotates,  as  do  all  the 
larger  bodies  of  the  solar  system ;  but  the  period  of  rotation 
is  not  exactly  known,  though  it  is  somewhere  between  25 
and  26|-  days.  Strangely  enough,  the  period  of  rotation 
appears  to  vary  according  to  the  latitude.  The  second  mo- 
tion is  one  in  which  the  entire  solar  system  is  engaged ;  but 
the  amount  and  exact  nature  of  this  is  not  known.  The  sys- 
tem is  moving  through  space  at  an  unknown  rate,  toward 
the  constellation  Hercules. 

The  Planets. — Mercuri/,  the  smallest  of  the  planets,  is  nearest 
to  the  sun,  on  the  average  being  about  35,750,000  miles  from 
it  (Fig.  6).     The  diameter  is  a  little  more  than  one-third 


Fig.  5. 
Sun  spots,  1872. 


Mart         Jujyiter  Saturn  Vranug  ^ejituno 

*f+HH 1 1 1 »  ' 


u 

**  ■U.t'tu.th 


Fig.  (5. 
Diagram  to  show  the  relative  distances  of  the  various  planets  from  the  sun. 

that  of  the  earth  (or  2992  miles),  and  it  rotates  on  its  axis 
in  about  24  hours,  while  it  revolves  around  the  sun  once 
in  about  88  days.  We  know  little  concerning  the  condi- 
tions on  this  planet. 


THE  EARTH  AS  A   PLANET. 


^arth 


The  next  body  outside  of  Mercury  is  Venus,  the  most 
brilliant  of  planets.  It  is  almost  the  same  size  as  the 
earth,  being  in  reality  about  250  miles  less  in  diameter 
(7660  miles)  (Fig.  7).  Some  observers 
think  that  they  have  detected  a  rotation 
with  a  period  of  a  little  more  than  24 
hours ;  but  this  is  doubted  by  most 
astronomers.  The  period  of  revolution 
is  considerably  less  than  ours,  or  about 
225  days.  It  appears  quite  certain  that 
there  is  an  atmosphere  upon  this  planet, 
and   so  far   as  we   can   tell,  it   closely 

resembles  ours.     No  satellite  is  known  Diao-ram  to  show  the  rela- 
te exist.  tive  size  of  the  smaller 

Outside  of  the  earth,  which  is  the 
next  planet  in  the  solar  system,  comes  Mars,  which  next 
to  Mercury,  is  the  smallest  of  the  planets,  having  a  diameter 
of  but  little  more  than  4200  miles.  Its  time  of  rotation  is 
a  little  over  24|-  hours,  and  its  revolution  about  the  sun 
is  accomplished  in  nearly  687  days.  Its  mean  distance  from 
the  sun  is  141,000,000  miles.  The  axis  of  Mars  is  inclined 
about  27°  to  the  plane  of  its  orbit,  which  is  about  4°  more 
than  the  inclination  of  the  earth's  axis.  There  are  two  tiny 
satellites,  one  less  than  10  miles  in  diameter,  the  other 
perhaps  twice  that  size ;  and  the  latter  is  not  more  than 
4000  miles  from  the  surface  of  the  planet,  about  which 
it  revolves  in  a  period  of  7  h.  39  m. 

Jupiter,  the  largest  of  planets  (Fig.  4),  has  a  mass  greater 
than  that  of  all  the  others  combined,  the  mean  diameter  being 
about  86,000  miles ;  but  the  diameter  at  the  equator  is  fully 
5000  miles  greater  than  that  at  the  poles.  The  volume 
of  Jupiter  is  about  1300  times  that  of  the  earth.  On  the 
average,  the    distance  from   the    sun   is   about   480,000,000 


10  PHYSICAL   GEOGBAPHY. 

miles,  and  it  takes  nearly  12  years  for  it  to  make  a  revolu- 
tion about  the  sun.  The  time  of  rotation  is  a  very  little 
over  9  h.  bb  m. 

It  is  evident  that  what  we  see  with  the  telescope  is  not 
the  surface  of  the  planet,  but  a  dense  atmosphere  of  some 
form  of  cloud.  Therefore  we  have  no  means  of  knowing 
what  the  actual  condition  of  Jupiter  is,  though  we  may  infer 
that  the  planet  is  still  heated,  and  that  the  clouds  which 
we  see  are  the  result  of  this  heated  condition.  Four  moons 
revolve  about  Jupiter,  the  most  distant  being  1,162,000  miles 
from  the  planet,  while  the  nearest  is  only  a  little  farther 
away  than  our  moon  is  from  us. 

Next  beyond  Jupiter  is  Saturn^  the  second  largest  of  the 
solar  planets.  Its  distance  is  881,000,000  miles  from  the 
sun,  around  which  it  revolves  in  about  29J  years,  while  it 
rotates  upon  its  axis  in  10  h.  14  m.^  This  planet  has  eight 
moons ;  but  the  most  remarkable  feature  connected  with  it, 
is  its  surrounding  pair  of  flattened  rings,  whose  inner  diame- 
ter is  100,000  miles.  These  rings  consist  of  many  separate 
bodies. 

As  the  distance  from  the  earth  increases,  our  knowledge 
of  the  members  of  the  solar  system  becomes  less  accu- 
rate. Hence,  since  its  mean  distance  from  the  sun  is  fully 
1,771,000,000  miles,  Uranus  is  scarcely  known.  It  revolves 
about  the  sun  once  in  84  years,  but  its  period  of  rotation 
is  not  known.     There  are  four  satellites. 

Until  1846  no  other  large  planet  was  known;  but  as  a 
result  of  prediction,  Neptune  was  discovered  in  that  year. 
The  discovery  of  this  planet  is  one  of  the  most  remarkable 
proofs  of  the  accuracy  of  the  theory  of  gravitation;  for  it 

1  It  will  be  noticed  that  as  the  distance  from  the  sun  increases,  the  time 
required  for  a  revolution  also  increases,  while  the  period  of  rotation  rapidly 
decreases. 


THE  EABTH  AS  A   PLANET.  11 

was  determined  by  irregularities  in  the  movement  of  Uranus, 
that  another  planet  must  exist  outside  of  its  orbit;  and  after 
careful  calculations,  the  place  where  this  planet  could  be 
found  was  predicted,  and  Neptune  was  discovered  at  a  mean 
distance  of  2,775,000,000  miles  from  the  sun.  One  moon 
has  been  detected. 

Asteroids.  —  In  the  year  1801,  a  small  planet  known  as 
Ceres  was  discovered  in  the  space  between  Mars  and  Jupiter. 
Since  that  time  about  400  other  smaller  bodies  have  been 
found  in  the  same  general  region.  In  no  cases  have  these 
small  planets  a  diameter  greater  than  520  miles,  while  the 
smallest  that  have  been  discovered  have  diameters  of  less 
than  40  miles.  Their  movement  through  space  is  some- 
what irregular;  and  there  have  been  many  speculations  con- 
cerning their  origin,  though  as  yet  no  satisfactory  explana- 
tion has  been  advanced. 

The  Earth.  —  While  cold  at  the  surface,  we  have  many 
reasons  for  believing  that  the  interior  of  the  earth  is  highly 
heated.  Proof  of  this  is  found  in  the  facts  that  at  the 
surface,  volcanoes  emit  quantities  of  molten  rock  which  come 
from  below,  and  that  in  all  deep  mines  and  well-borings  the 
temperature  of  the  rocks  is  found  to  increase  at  a  moderately 
uniform  rate,  on  the  average  1°  for  about  every  50  or  60 
feet  of  descent.  If  this  rate  of  increase  continues,  the  rocks 
at  a  depth  of  less  than  100  miles  are  so  hot  that  they  would 
be  molten  under  the  conditions  which  exist  at  the  surface. 

It  was  once  believed  that  the  interior  of  the  earth  was  in 
a  molten  condition,  and  that  the  solid  surface  was  merely 
a  crust  resting  upon  this  liquid  sphere;  but  many  facts  now 
lead  us  to  the  belief  that  the  interior  is  at  least  as  rigid  as 
steel.  The  proof  of  this  has  been  furnished  by  the  studies  of 
physicists  and  astronomers.  At  present  we  are  forced  to  the 
belief,  that  although  highly  heated,  the  rocks  in  the  interior 


12 


PHYSICAL   GEOGRAPHY. 


of  the  earth  are  prevented  from  melting  by  the  great  pres- 
sure of  the  overlying  layers;  and  by  this  theory  we  are  able 
to   satisfactorily   account   for    all    of    the   phenomena   that 


Diagram  illustrating  the  cause  of  seasons. 

formerly  seemed   to   demand   the   explanation   of   a   liquid 
interior. 

The   earth   is   engaged   in   a   number   of    movements   in 
space.     It  revolves  around  the  suu  in  about  365J  days,  in  an 


THE  EARTH  AS  A  PLANET.  13 

orbit  which  is  nearly  a  circle;  but  instead  of  being  actually 
a  circle  with  the  sun  at  its  center,  the  orbit  is  really  an 
ellipse  with  the  sun  at  one  of  the  foci.  Therefore,  in  the 
course  of  its  revolution,  the  earth  is  at  one  time  farther  from 
the  sun  than  in  the  opposite  season,  the  distance  now  vary- 
ing between  91,000,000  and  94,000,000  miles,  with  an  average 
distance  of  about  92,750,000  miles. 

During  the  revolution,  the  earth  rotates  about  one  of  its 
diameters,  which  we  call  the  axis,  and  this  rotation  occu- 
pies a  little  less  than  24  hours  (23  h.  56  m.),  or  one 
day.  This  rotation  causes  the  familiar  alternation  of  day 
and  night;  and  if  the  earth's  axis  were  at  right  angles  to  the 
plane  of  revolution,  the  day  and  night  would  be  equal  in 
length;  but  since  it  is  inclined  to  this  plane  at  an  angle  of 
23°  27',  the  relative  length  of  day  and  night  varies  from 
day  to  day.  Indeed,  tlie  seasons  themselves  depend  upon 
this  inclination  of  the  poles  (Fig.  8);  for  in  the  course 
of  a  revolution,  the  pole  is  always  pointed  toward  a  certain 
part  of  the  heavens ;  and  as  the  earth  moves  about  the  sun, 
the  northern  hemisphere  alternately  faces  and  is  turned  away 
from  the  sun.  When  turned  toward  the  sun,  the  summer 
season  is  caused,  and  when  turned  away  from  it,  the  winter 
season  results,  because  the  solar  rays  then  fall  less  vertically 
upon  the  hemisphere,  and  the  length  of  the  day  is  shorter. 
Between  these  two  opposite  seasons  we  have  spring  and 
autumn. 

The  Moon. —  This,  the  nearest  to  our  earth  of  all  the 
heavenly  bodies,  has  an  average  distance  of  about  240,000 
miles,  and  a  diameter  of  2160  miles  (Fig.  9).  Since  the  path 
of  the  moon  about  the  earth  is  an  ellipse  with  the  earth  at 
one  of  the  foci,  the  distance  varies;  but  it  is  rarely  more 
than  253,000  miles  nor  less  than  227,000  miles  distant. 
When  farthest  from  the  earth  it  is  said  to  be  in  Apogee, 


14 


PHYSICAL   GEOGRAPHY, 


Fig.  9. 

The  relative  size  of 
earth  and  moon. 


and  when  nearest  in  Perigee ;  and  once  in  every  revolution 

Apogee  and  Perigee  are  reached. 

Aside  from  those  it  makes  in  company  with  the  earth,  its 

two  important  movements  in  space  are  a  revolution  around 

the  ea^th  and  a  rotation  about  an  axis,  both 
of  these  movements  occurring  in  the  same 
period  of  time,  or  29J  days.  Therefore 
one  side  of  the  moon  is  never  seen  from  the 
earth.  Also,  as  a  result  of  this  condition, 
the  length  of  the  lunar  day  is  29J^  of  our 
days ;  and  therefore  at  the  lunar  equator 
the  sun  shines  steadily  for  nearly  15  days 
and    is    absent    an    equal   length  of  time. 

Under  these  conditions  the  surface  of  the  moon  is  warmed 

during  the  long  day,  and  at  night  becomes  cooled  down  to 

temperatures     which 

are  perhaps  as  low  as 

-  200°. 

There  is  no  atmos- 
phere and   no    ocean 

on    the    moon ;    and 

the  only  change  upon 

the  surface  seems  to 

be  that  between  con- 
ditions  of    heat   and 

cold,    and    light   and 

darkness.      It  emits 

an  almost  impercepti-  Fig.  lo. 

ble  amount  of  radiant  Lunar  craters,  the  largest  being  Gassendi. 

energy,  and  the  light  from  the  moon  is  reflected  sunlight,  i 
As  a  result   of  the  careful   telescopic   study  of  the  moon, 

1  Direct  light  from  the  sud  is  600,000  times  as  strong  as  that  which  is 
reflected  from  the  moon. 


THE  EARTH  AS  A  PLANET.  15 

astronomers  have  been  able  to  map  many  of  the  details  of  lunar 
topography,  with  considerable  accuracy,  and  even  to  measure 
mountain  heights.  While  there  are  other  striking  topo- 
graphic features,  the  most  notable  thing  about  the  lunar  land- 
scape is  the  great  number  of  crater-like  mountains,  which  bear 
a  certain  resemblance  to  the  volcanoes  on  the  earth's  surface, 
excepting  that  many  of  them  are  of  immense  size  (Fig.  10). 
Comets,  Shooting  Stars  and  Meteors.  —  Aside  from  those 
described,  which  may  be  considered  the  normal  members  of 
the  solar  system,  there  are  other  heavenly  bodies  which 
do  not  appear  to  be  regular  parts  of  the  system.  The 
strangest  of  these  are  comets.  Some  500  of  these  have 
been  recorded  as  visible  to  the  naked  eye ;  and  in  addition, 
over  200  have  been  detected  by  the  aid  of  the  telescope,  some 
of  these  being  millions  of  miles  in  length.  When  near  the 
sun,  they  usually  have  a  relatively  dense  head  and  a  vaporous 
tail,  through  which  stars  are  visible  (Fig.  11).  Some  have 
regular  elliptical  orbits,  and 
their  time  of  appearance  can 
be  closely  calculated ;  but 
the  orbits  of  others  are  ap- 
parently parabolas,  so  that 
if  they  ever  return  to  the 
solar  system,  it  is  only  after 
long  periods  of  time  have 
elapsed,  and  after  having 
made  a  journey  far  beyond  ^^^^  ^7«_^^>^^._  ^^_ 

the  outermost  limits  of  the 

solar  system.  Perhaps  these  may  be  mere  wanderers  through 
space,  which  after  one  visit  to  the  solar  system,  depart  never 
to  return  again.  What  they  are,  whence  they  came,  whither 
they  are  going,  or  what  relation  they  bear  to  the  solar  sys- 
tem, is  still  an  unsolved  mystery. 


16 


PHYSICAL   GEOGRAPHY, 


0,hi\  of  August   Meteors 


Comets  have  an  added  interest  to  us,  from  the  fact  that 
some  shooting  stars  and  meteors  seem  to  be  remnants  of 
comets,  which  at  some  former  time  have  crossed  the  orbit 
of  the  earth.  Thus  the  November  meteorites  are  due  to  the 
fact  that  in  its  movement  around  the  sun  the  earth  en- 
counters particles  that  are  left  in  the  trail  of  a  comet  (Tem- 
pel's)  which  has  a  period  of  revolution  of  about  thirty-three 
years;  and  the  August  meteors  (Fig.  12)  appear  to  have 
a  similar  origin. 

Meteors  and  shooting  stars  (meteors  are  large  shooting 
stars)  enter  the  earth's  atmosphere  at  a  high  rate  of  speed, 

and  are  burned  up 
in  the  higher  layers 
of  the  atmosphere, 
often  at  an  eleva- 
tion as  great  as  100 
miles  from  the  sur- 
face of  the  earth. 
This  burning  is 
the  result  of  fric- 
tion with  the  air,  which  produces  a  high  heat,  because  in 
addition  to  the  movement  of  the  meteor,  there  is  often 
added  the  motion  of  the  earth  itself,  which  is  about  98,000 
feet  a  second.  Hence  in  small  bodies,  the  burning  is  almost 
instantaneous ;  but  some  of  the  larger  meteors  pass  entirely 
through  the  atmosphere,  and  reach  the  earth's  surface. 

A  study  of  these  rather  rare  meteorites,  reveals  to  us  the 
very  interesting  fact  that  no  new  element  exists  in  them ; 
and  therefore  we  may  fairly  conclude  that  the  elements 
composing  comets  are  the  same  as  some  of  those  which  make 
up  the  earth's  crust.  In  watching  the  heavens  at  night, 
scarcely  an  Iiour  can  pass  without  noticing  shooting  stars ; 
and  since  the  same  would  probably  be  true  of  the  day  if  we 


Fig.  12. 

Orbit  of  the  second  comet  of  1862. 


TEE  EARTH  AS  A  PLANET. 


17 


could  then  see  tliem,  we  conclude  that  there  are  immense 
numbers  of  these  bodies  in  the  space  through  which  the  earth 
travels. 

The  Stellar  System. —  Far  away  in  space,  many  times 
farther  than  the  sun  is  from  us,  innumerable  stars  are 
scattered.  Already  many  thousands  are  known,  and  it  is 
estimated  that  over  30,000,000  are  visible  with  the  telescope. 
Like  the  sun,  they  emit  an  energy  which  produces  both 
light  and  heat ;  and  it  is  very  probable  that  many,  if 
not  all,  have  planetary  bodies  revolving  about  them. 
One  satellite,  that  belonging  to  Sirius,  has  already  been  dis- 
covered ;  and  some  double 
stars  are  known  to  be  re- 
volving about  a  common 
center  of  gravity.  The 
distance  between  the  stars, 
and  even  between  the  earth 
and  the  nearest  stars,  is  im- 
mense, and  in  most  cases  in- 
calculable. If  each  star  is 
a  sun  with  accompanying 
planets,  and  if  each  of  these 
suns  is  as  far  from  its  near- 
est stellar  neighbors  as  we 
are  from  ours,  the  immensity 
and  grandeur  of  the  system 
transcends  our  imagination. 

The  stars  are  arranged  in 
a  disc-like  belt,  the  greatest 

diameter  of  which  is  in  the  direction  of  the  Milky  Way. 
At  right  angles  to  this  there  is  a  zone  of  abundant  nebulce, 
(Fig.  13),  although  these  strange  bodies  are  not  absent  from 
other  parts  of  the  heavens.       Some  have  conjectured  that 


Fig.  13. 
Andromeda  nebula,  from  a  drawing. 


18  PHYSICAL   GEOGRAPHY, 

nebulae  are  other  stellar  systems,  so  distant  from  us  that 
the  individual  members  cannot  be  separated  by  our  tele- 
scopes ;  but  the  spectroscope  seems  to  show  that  they  are 
bodies  of  glowing  gas,  and  this  has  an  important  bearing 
upon  the  nebular  hypothesis,  which  we  soon  discuss.  One 
very  important  thing  concerning  both  stars  and  nebult«,  is 
that  the  spectroscope  has  detected  in  them  many  of  the 
elements  which  we  find  upon  the  earth. 

A  question  of  very  deep  interest,  is  whether  the  stars  form 
a  great  system  in  which  the  individual  members  are  inter- 
related, as  is  the  case  among  the  members  of  the  solar 
system  ?  Unfortunately,  in  the  present  state  of  science,  we 
are  unable  to  return  a  definite  answer  to  this  question. 

Symmetry  of  the  Solar  System. — In  theorizing  upon  a 
basis  of  known  facts  we  must  confine  ourselves  to  the  solar 
system  ;  and  it  is  interesting  to  note  the  wonderful  symmetry 
of  arrangement  and  the  beautiful  order  which  exists  here. 
Throughout  the  entire  system,  the  law  of  gravitation  prevails 
and  governs  the  movements  of  all  the  bodies,  each  member 
attracting  the  other  in  direct  proportion  to  the  product  of 
the  masses  and  inversely  proportional  to  the  square  of  the 
distance.  The  regular  members  of  the  system  are  all  nearly 
spherical,  and  they  rotate  about  an  axis  and  revolve  in  an 
orbit  which  is  nearly  circular.  In  direction  of  rotation  and 
revolution  there  is  a  marked  uniformity,  as  there  is  also  in 
the  plane  of  revolution. 

All  of  these  regularities  of  behavior,  take  place  notwith- 
standing the  fact  that  immense  distances  separate  the  various 
bodies,  and  that  this  space  is  practically  void.  We  can  form 
no  accurate  conception  of  these  immense  distances ;  but  the 
following  quotation  from  Newcomb's  Astronomy  furnishes 
some  idea  of  this  :  — 

"To   give   an   idea   of  the  relative   distances,  suppose  a 


THE  EARTH  AS  A  PLANET.  19 

voyager  through  the  celestial  spaces  could  travel  from  the 
sun  to  the  outermost  planet  of  our  system  in  twenty-four 
hours.  So  enormous  would  be  his  velocity,  that  it  would 
carry  him  across  the  Atlantic  Ocean,  from  New  York  to 
Liverpool,  in  less  than  a  tenth  of  a  second  of  the  clock. 
Starting  from  the  sun  with  this  velocity,  he  would  cross  the 
orbits  of  the  inner  planets  in  rapid  succession,  and  the  outer 
ones  more  slowly,  until,  at  the  end  of  a  single  day,  he  would 
reach  the  confines  of  our  system,  crossing  the  orbit  of  Nep- 
tune. But,  though  he  passed  eight  planets  the  first  day,  he 
would  pass  none  the  next,  for  he  would  have  to  journey 
eighteen  or  twenty  years,  without  diminution  of  speed, 
before  he  would  reach  the  nearest  star,  and  would  then  have 
to  continue  his  journey  as  far  again  before  he  could  reach 
another.  All  the  planets  of  our  system  would  have  vanished 
in  the  distance,  in  the  course  of  the  first  three  days,  and  the 
sun  would  be  but  an  insignificant  star  in  the  firmament." 

The  sun  in  the  center  of  the  solar  system  is  a  true  star,  in 
many  respects  like  the  others  which  dot  the  firmament. 
This  being  the  case,  may  we  not  fairly  speculate  as  to  the 
possibility  of  other  worlds  and  systems  like  our  own,  far 
away  in  space,  even  to  the  outermost  limits  which  can  be 
reached  .by  the  human  vision ;  and  if  this  be  so,  how  vast  is 
the  universe,  and  how  insignificant  the  small  cold  body  of 
matter  upon  which  we  dwell ! 

The  Nebular  Hypothesis.  —  Before  many  facts  concerning 
the  universe  were  known,  the  philosopher  Kant  proposed  a 
hypothesis  to  account  for  the  origin  of  the  solar  system  ;  and 
later,  Herschel  and  Laplace  proposed  an  explanation  which 
in  many  respects  was  like  that  of  Kant.  We  know  this 
explanation  under  the  name  of  the  nebular  hypothesis. 

By  this  it  is  assumed  that  the  space  occupied  by  the 
members  of  the  solar  system,  and  probably  even  to  a  con- 


20  PHYSICAL   GEOGRAPHY. 

siderable  distance  beyond  this,  was  occupied  by  a  nebulous 
mass  of  highly  heated  vapor.  It  is  one  of  the  laws  of  nature 
that  radiant  energy  passes  from  warmer  to  colder  bodies,  and 
that  by  this  radiation  a  contraction  and  condensation  neces- 
sarily follow.  This  nebulous  mass,  composed  of  all  the  ele- 
ments which  now  enter  into  the  composition  of  the  various 
members  of  the  solar  system,  during  the  process  of  cooling 
separated  into  rings  which  were  the  parents  of  the  several 
planets.  As  the  mass  lost  heat  and  began  to  condense  and 
contract,  the  materials  began  to  accumulate  about  some 
denser  part  of  these  rings,  the  accumulations  about  these 
denser  portions  being  determined  by  the  fact  that  gravita- 
tive  action  was  stronger  there  than  elsewhere. 

As  a  result  of  this  accumulation  about  centers,  the  original 
nebulous  mass  became  broken  up  into  several  smaller  masses 
of  similar  nature  ;  and  by  a  continuation  of  the  process  other 
rings  were  thrown  off,  out  of  which  the  satellites  were 
formed.  Original  motion  about  a  central  portion  of  the 
nebula  has  naturally  been  inherited  and  is  now  indicated 
by  the  movements  of  the  bodies  in  the  solar  system.  The 
cooling  of  these  bodies  is  still  in  progress,  and  different 
members  of  the  system  have  reached  different  stages. 

Verification  of  the  Nebular  Hypothesis. — While  we  cannot 
state  that  this  theory  is  definitely  proven,  many  facts  point 
to  its  truth  as  a  general  explanation  of  the  solar  universe. 
For  instance,  it  would  account  for  the  fact  that  the  planets 
move  about  the  sun  in  a  common  direction,  and  that  the 
planes  of  revolution  are  nearly  the  same  in  the  different 
planets  (the  inclination  in  no  case  being  more  than  a  few 
degrees).  This  similarity  also  extends  even  to  the  satellites  ; 
and  the  rotation  of  the  bodies  whose  rotation  has  been 
determined  has  the  same  kind  of  uniformity.  All  of  the 
orbits    of    the   members   of   the    solar    system    are    ellipses 


THE  EABTH  AS  A   PLANET.  21 

approaching  a  circle.  This  together  with  the  uniform  action 
of  gravitation  suggests  a  common  origin. 

The  fact  that  all  the  bodies  regularly  belonging  to  the 
solar  system  are  nearly  spherical  in  form  is  suggestive ;  and 
this  form  can  readily  be  accounted  for  if  the  bodies  were 
once  liquid.  A  former  liquid  condition  is  suggested  by  the 
fact  that  those  bodies  which  are  well  known,  all  have  a 
larger  diameter  at  the  equator  than  at  the  poles,  although 
it  is  true  that  this  may  be  explained  in  other  ways.  Then 
also,  signs  of  heat  are  plainly  seen  in  some  of  the  mem- 
bers of  the  solar  system ;  and  in  the  smaller  bodies  these 
signs  are  less  apparent.  Thus  the  sun  is  highly  heated ; 
Jupiter,  Saturn,  and  other  of  the  outer  planets  show  signs 
of  considerable  heat ;  the  earth  is  cold  at  the  surface,  and 
hot  in  the  center ;  Mars,  Venus,  and  Mercury  are  cold  at  the 
surface ;  and  the  moon  appears  to  be  entirely  cold. 

Upon  the  nebular  hypothesis,  we  should  expect  that  the 
density  of  the  members  of  the  solar  system  would  increase 
from  the  outer  bodies  toward  the  center ;  and  this  actually 
is  the  case,  the  only  exceptions  being  the  easily  explained 
cases  of  Saturn  and  the  sun.  There  are  other  reasons  for 
believing  in  the  nebular  hypothesis.  So  far  as  we  may 
judge  from  the  results  of  spectroscopic  study  and  from 
the  examinations  of  meteorites  that  have  fallen  upon  the 
earth,  the  bodies  in  the  solar  system  are  composed  of  the 
same  elements  as  those  which  make  the  earth ;  and  this  sug- 
gests that  they  have  been  made  from  the  same  original  mass. 

Far  away  in  space,  beyond  the  solar  system,  we  even  find 
nebulous  masses  of  gas  which  are  exactly  like  those  out  of 
which  the  solar  system  is  believed  to  have  been  made ;  and 
in  some  of  these  nebula)  the  condensation  into  planetary 
bodies  appears  to  be  in  progress  (Fig.  13).  Nearly  every 
gradation  has  been  found  between  this  kind  of  nebula  and 


22  PHYSICAL   GEOGBAPUY. 

that  which  is  apparently  one  mass  of  glowing  gas.  It  is 
not  improbable  that  even  now  other  worlds  are  in  process 
of  formation  in  the  far  distant  regions  of  space. 


REFERENCE   BOOKS.i 

Newcomb. — Popular  Astronomy  (school  edition).  Harper  Brothers,  New 
York.  Seventh  edition,  1894.  8vo.  Published  also  in  larger  form. 
School  edition,  $1 .30  ;  larger  book,  $2. 50.   (General  and  quite  elementaiy. ) 

Lockyer.  —  Elementary  Lessons  in  Astronomy.  Macmillan  &  Co.,  New 
York.     Svo.     $1.25.     (General  and  elementary.) 

Chambers.  —  Handbook  of  Descriptive  and  Practical  Astronomy.  Mac- 
millan «&  Co.,  New  York.  Fourth  edition,  1889.  Svo.  Three  volumes. 
Vol.  I.,  $5.25  ;  Vol.  II.,  $5.25 ;  Vol.  III.,  $3.50.  (Large  and  comprehen- 
sive.) 

Proctor  and  Ranyard.  —  Old  and  New  Astronomy.  Longmans,  Green,  & 
Co.,  New  York,  1892.     Svo.    $12.00.     (Complete  and  well  illustrated.) 

Young. — The  Sun.  International  Scientific  Series.  Appleton  &,Co.,  New 
York,  1893.     12mo.     $2.00. 

Lockyer. — The  Chemistry  of  the  Sun.  Macmillan  &  Co.,  New  York,  1887. 
Svo.     $4.50. 

Nasmyth  and  Carpenter. — The  Moon.  Murray,  London  (Scribner,  New 
York  agents),  1885.     Svo.     $8.40.     (Many  remarkable  photographs.) 

Nelson.  —  The  Moon.  Longmans,  Green,  &  Co.,  New  York,  1876.  Svo. 
$10.00.     (Well  illustrated.) 

Lockyer. — The  Meteoritic  Hypothesis.  Macmillan  &  Co.,  New  York, 
1890.  Svo.   $5.25.  (Suggestion  of  modification  of  the  nebular  hypothesis.) 

Scheiner  (translated  by  Frost).  —  A  Treatise  on  Astronomical  Spec- 
troscopy.    Ginn  &  Co.,  Boston,  1894.     Svo.     $5.00. 

*  In  giving  the  publisher's  name,  the  real  publishing  house  Is  often  not  mentioned. 
Wherever  possible  American  houses  are  given,  and  since  some  of  these  act  as  agents  for 
£uro|>ean  houses,  the  name  of  the  agent  will  at  times  appear  in  the  place  of  the  English 
publisher. 


CHAPTER   II. 


THE  ATMOSPHERE. 


600 
Sfiles 


General  Statement.  —  Outside  of  the  solid  earth,  and  ex- 
tending to  a  distance  of  several  hundred  miles  above  it,  is  a 
gaseous  envelope,  which  we 
know  as  the  atmosphere  (Fig. 
14) .  Its  density  decreases  from 
the  surface  of  the  earth  toward 
the  upper  portions;  and  at  a 
height  of  five  miles  it  is  very 
much  rarefied.  That  it  ex- 
tends to  this  great  height  is 
shown  by  the  fact  that  meteors 
become  white  hot  by  friction 
with  it,  even  at  a  greater  dis- 
tance than  this  from  the  earth. 
Fully  one-half  of  the  mass  of 
the  atmosphere  is  within  four 
miles   of    the   surface   of   the 


The  earth  with  its  atmospheric  envel- 
ope, drawn  to  scale. 


earth ;  and  two-thirds  of  it  is  within  six  miles  of  the  surface 
(Fig.  15). 

The  atmosphere  is  composed  almost  entirely  of  two  gases, 
nitrogen  and  oxygen,  in  the  proportion  of  about  79  to  21. 
These  gases  are  not  in  chemical  combination,  but  are 
mechanically  mixed.  Nitrogen  is  a  very  inert  element,  while 
oxygen  is  active  in  the  production  of  many  changes,  and  from 

23 


24  PHYSICAL   GEOGRAPHY. 

this  standpoint  the  nitrogen  of  the  air  may  be  considered  as 
an  adulterant  of  the  active  oxygen.  In  addition  to  these 
gases  there  is  a  comparatively  small  amount  (about  0.03  per 
cent)  of  carbonic  acid  gas,  the  percentage  varying  some- 
what according  to  the  location.  Its  percentage  increases  in 
the  vicinity  of  volcanoes  and  large  cities.^ 

Beside  these  three  gases  there  are  minor  and  variable  quan- 
tities of  other  substances ;  but  of  these,  only  two,  water  vapor 
and  dust  particles,  are  of  sufficient  general  importance  for 
consideration  here.     The    term    ^''dusV    includes    a    great 


t   » 


■■■■■  •  .'•:••  :;-^'*-2 -v^ ^£/vi^^•^^ Av--v:^:^.<^^;j^^  :••■> •; ■-. ■■/■: -.■■■: ; • .-. ■ .- . -  ■■•■  ■ 

^^^^  Fig.  15.  ^^''^sSiyi^  ^    

Diagram  to  illustrate  decrease  in  density  of  the  atmosphere. 


variety  of  substances,  such,  for  instance,  as  microbes,  smoke 
particles,  and  true  dust,  which  is  borne  into  the  air  by  the 
winds.  It  seems  certain  that  dust  is  of  much  importance  in 
the  formation  of  rain  and  fog. 

Water  is  readily  evaporated,  and  hence  at  all  times  there  is 
some  water  vapor  in  the  air;  but  the  amount  depends  upon  a 
variety  of  circumstances,  chiefly  the  temperature  of  the  air 
and  the  presence  or  absence  of   bodies  of  water.      With  a 

1  While  this  book  is  in  preparation,  the  discovery  of  a  new  constituent  of 
the  atmosphere  is  announced.  This,  which  is  called  argon,  may  be  a  new 
element,  but  it  is  now  too  early  to  state  anything  definite  about  this  sub- 
stance. 


THE  ATMOSPHERE.  25 

given  amount  of  moisture,  the  higher  the  temperature,  the 
greater  the  rate  of  evaporation;  but  even  at  temperatures 
below  freezing-point  small  quantities  of  water  vapor  may  be 
present. 

The  atmosphere  is  of  great  importance  in  many  respects. 
It  distributes  the  light  which  comes  to  us  from  the  sun.  It  is 
set  in  motion  by  the  solar  energy,  and  by  this  means  distrib- 
utes heat  over  the  earth.  As  a  result  of  the  effect  of  solar 
heat  upon  the  atmosphere  a  great  variety  of  phenomena,  such 
as  winds,  storms,  clouds,  etc.,  are  produced.  These  cause 
many  changes  upon  the  surface  of  the  earth,  and  directly  and 
indirectly  the  air  makes  the  earth  a  place  fit  for  habitation. 

Light.  —  We  obtain  light  from  several  sources,  —  the  sun, 
the  stars,  and  the  moon  and  planets.  Light  from  the  latter 
source  is  merely  reflected  sunlight,  and  it  is  small  in  amount. 
That  which  comes  from  the  stars  is  radiated  from  them 
directly,  but  it  also  is  insignificant  in  comparison  with  that 
received  from  the  sun. 

Solar  light,  when  it  reaches  the  lower  layers  of  the  atmos- 
phere, produces  the  impression  upon  the  eye  which  we  know 
as  white ;  but  it  has  been  shown  that  it  probably  has  a  bluish 
tinge  befora  its  passage  through  the  air.  According  to  the 
undulatory  theory,  light  passes  through  the  space  between 
us  and  the  sun  at  a  very  rapid  rate  in  the  form  of  a  series  of 
waves  of  ether.  It  is  made  up  of  many  waves  of  different 
lengths,  the  combination  of  which  gives  white.  When 
separated,  these  appear  as  different  colors,  and  in  the  rain- 
bow we  recognize  seven  primary  colors  with  intermediate 
hues.  The  violets  and  blues  have  the  shortest  vibrations, 
and  the  yellows  and  reds  the  longest.  As  a  result  of  the 
effect  of  the  atmosphere  upon  these  parts  of  white  light 
many  optical  phenomena  are  produced. 

If  there  were  no  atmosphere,  the  earth's  surface  would  be 


26 


PHYSICAL   GEOGRAPHY, 


illuminated  only  where  the  direct  rays  of  the  sun  fell.  The 
atmosphere  serves  to  diffuse  light  and  to  render  the  darkness 
of  shadows  less  intense.  This  diffusion  of  light  in  large 
measure  depends  upon  the  amount  of  solid  or  liquid  impuri- 
ties in  the  air.  In  its  passage  through  the  air,  certain  of  the 
rays  are  diffused  more  readily  than  others  by  the  process  of 
selective  scattering.  It  is  those  rays  that  have  the  shortest 
wave  lengths  that  are  thus  scattered ;  and  hence  it  is  that 
the  sky  is  ordinarily  blue.  The  intensity  of  the  blue  is  great- 
est when  coarse  dust  impurities  are  least  abundant,  as  is  the 
case  when  the  air  is  clear  and  dry.  If  dust  particles  happen 
to  be  very  abundant,  even  the  coarser  rays  of  yellow  light 
may  be  scattered  ;  and  under  rare  conditions  of  very  smoky 
air  the  entire  sky  may  assume  a  brassy  color.  Since  the 
light  is  obliged  to  travel  through  a  greater  distance  of  air 
near  the  time  of  sunset  than  in  midday,  the  color  of  the 
western  sky  in  the  late  afternoon  is  often  yellow,  while  that 

of    midday    was    a 
,o8p^^^^   \         dull  hazy  blue  (Fig. 

16). 

Among  the  most 
beautiful  of  light 
effects  in  the  atmos- 
phere is  that  of  the 
sunset  colors^  which 
are  due  to  the  scat- 
tering of  the  waves 

Diagram  to  show  that  the  sun's  rays  pass  through  a  ...i^  •  i.  u  ^  ^ra  +  It  o 
greater  thickness  of  atmosphere  at  sunset  and  sun-  ^  ^^  ^  ^  ^^  n  a  V  e  t  n  e 
rise  than  at  midday.  (Tliickness  of  atmosphere  smaller  lengths.  As 
greatly  exaggerated).  ,,      £  ^\^'      ^i 

a  result  oi  this  the 
coarser  yellows  and  reds  come  to  us,  the  reason  for  the  scat- 
tering being  the  fact  that  the  light  at  the  time  of  sunset  and 
sunrise  passes  through  a  great  thickness  of  air,  and  hence  the 


THE  ATMOSPHERE.    -  27 

waves  encounter  a  greater  number  of  dust  particles.  When 
the  atmosphere  contains  much  dust,  the  morning  and  evening 
colors  are  often  very  intense,  but  an  increase  in  the  quantity 
of  dust  beyond  a  certain  point  tends  to  dull  the  tints.  With 
clouds  in  the  horizon  at  sunset  or  sunrise,  these  colors  of  red 
and  yellow  are  often  reflected  in  infinite  variety  of  shade 
and  tint.  Other  phenomena,  such  as  the  twilight  arch,  the 
glow  and  the  afterglow,  are  associated  with  the  setting  of 
the  sun. 

Another  property  of  light  is  that  of  reflection^  and  as  a 
result  of  this  many  interesting  optical  effects  are  produced. 
The  light  of  the  moon  depends  upon  the  reflection  of  sun- 
light from  its  surface.  The  earth  also  reflects  light,  and 
this  is  one  of  the  reasons  for  the  illumination  of  places  that 
are  in  the  shadow  of  the  direct  rays  of  the  sun.  Other 
places  which  are  illuminated  reflect  some  of  their  light  to 
the  parts  that  are  in  shadow.  Clouds  also  reflect  the  light 
of  the  sun ;  and  on  summer  days,  when  great  banks  of 
clouds  rise  high  in  the  air,  their  surfaces  are  brilliantly 
illuminated  and  beautiful  cloud  effects  are  produced. 

Another  effect  of  reflection  is  the  mirage^  which  occurs 
when  the  air  near  the  surface  is  warmer  than  the  layers 
above  it,  and  when  the  reflection  from  this  warm  air  layer 
reaches  the  eye  of  the  observer.  It  often  gives  rise  to  an 
appearance  like  that  of  a  sheet  of  water ;  and  travelers  in 
desert  lands,  where  this  phenomenon  is  common,  are  often 
led  to  think  that  they  are  actually  approaching  a  lake.  One 
very  commonly  sees  such  an  appearance  as  this  at  the  sea  or 
lake  shore  when  distant  coasts  appear  to  rise  above  the  sur- 
face of  the  water.  It  sometimes  happens  that  light  is 
reflected  from  a  warm  layer  which  is  above  the  observer; 
and  then  the  objects  appear  upside  down.  This  "  looming," 
as  it  is  called,  is  particularly  common  in  Arctic  regions ;  and 


28  PHYSICAL   GEOGRAPHY. 

the  effect  produced  is  so  fantastic  and  wonderful  that  nearly 
all  Arctic  explorers  describe  it. 

The  rainbow  is  a  phenomenon  which  partly  depends  upon 
the  reflection  of  sunlight ;  but  it  is  chiefly  due  to  refraction, 
the  result  being  a  separation  of  the  several  components  of 
white  light  into  the  colors  of  the  spectrum.  Each  person 
sees  a  different  rainbow  even  though  two  observers  may 
stand  side  by  side.  The  cause  for  the  phenomenon  is  the 
effect  of  raindrops  which,  being  denser  than  the  air,  bend 
and  separate  the  rays  of  white  light  so  that  we  see  the 
component  colored  rays,  just  as  we  do  when  a  sunbeam  passes 
through  a  prism.  A  rainbow  is  often  produced  in  the  spray 
that  rises  at  the  base  of  a  waterfall,  and  at  the  distance  of 
only  a  few  yards  one  may  see  it  outlined  in  the  spray. 

Another  phenomenon  resulting  from  the  combined  action 
of  refraction  and  reflection  is  the  ring  of  light  or  halo  which 
often  surrounds  the  sun  or  moon  when  their  light  passes 
through  thin  hazy  clouds  in  the  upper  atmosphere.  These 
clouds  are  composed  of  ice  particles,  which  act  upon  the  light 
in  a  manner  analogous  to  the  effect  of  raindrops  in  the 
production  of  the  rainbow.  Very  remarkable  halos  are 
formed,  particularly  in  Arctic  regions,  where  the  air  is  often 
filled  with  minute  crystals  of  ice.  Sometimes  rings  of  light 
of  very  brilliant  colors  are  thus  produced.  The  interference 
with  light  resulting  from  the  presence  of  water  or  ice  in 
clouds  often  produces  a  ring  of  light  immediately  around 
the  sun  or  moon.  These  are  called  coronas,  and  they  are 
often  beautifully  colored,  the  colors  being  arranged  in  con- 
centric rings  with  the  red  on  the  outside. 

One  of  the  most  important  of  the  phenomena  of  light  is 
that  of  absorption.  Many  bodies,  such  as  pure  air  and  water, 
allow  most  of  the  rays  of  light  to  pass  through  them  with  little 
change,  and  such  bodies  are  called  transparent.     Other  sub- 


«k 


THE  ATMOSPHERE.  29 

stances  are  only  partially  transparent,  and  we  know  them 
under  the  name  of  translucent  bodies.  Still  others  which  we 
know  as  opaque  do  not  allow  any  light  to  pass.  Thus  objects 
have  a  red  color  when  they  reflect  a  greater  number  of  the 
red  than  of  the  other  rays ;  and  other  colors  are  produced  in 
the  same  way  by  the  absorption  of  different  proportions  of 
the  rays. 

Electricity  and  Magnetism.  —  There  are  certain  phenomena 
of  magnetism  in  the  earth  which  some  believe  to  exercise  a 
decided  influence  upon  the  atmosphere.  The  earth  is  a  great 
magnet,  and  the  region  of  greatest  magnetic  attraction  is 
near  Hudson's  Bay,  toward  which  the  needle  of  the  compass 
points  in  our  hemisphere.  This  may  be  called  the  north 
magnetic  pole.  The  magnetic  condition  of  the  earth  is  con- 
stantly changing,  both  in  small  daily  variations  and  in 
annual  changes,  as  well  as  in  variations  covering  many 
years.  Occasionally  there  are  magnetic  storms,  when  there 
is  a  disturbance  of  magnetic  instruments,  and  when  the 
aurora  sometimes  develops  in  wonderful  complexity  and 
weird  beauty.  This  is  some  electrical  effect  in  the  thin 
upper  atmosphere ;  but  our  knowledge  of  these  phenomena 
is  obscure. 

Electricity  is  produced  in  the  atmosphere  by  various 
causes,  and  it  is  nearly  always  present ;  but  only  rarely  does 
it  develop  sufficient  intensity  to  become  visible  to  the  eye. 
In  thunderstorms  and  tornadoes,  Avhen  the  air  is  in  violent 
commotion,  there  is  often  sufficient  electricity  to  cause  vivid 
discharges  from  one  cloud  to  another,  or  to  the  earth.  This 
lightning  is  an  interesting  phenomenon,  but  it  does  not  appear 
to  have  an  important  influence  in  the  formation  of  the  storms, 
being  really  a  result  of  them.  The  accompanying  sound 
is  often  changed  to  a  rumble  by  reverberation  and  echoes 
among  the  clouds,  and  between  them  and  the  earth.     Often 


30  PHYSICAL   GEOGRAPHY, 

in  violent  thunderstorms  the  air  is  filled  with  a  constant 
roar  of  thunder.  The  lightning  spark  or  bolt  is  sometimes 
a  single  large  spark,  or  it  may  divide  and  sub-divide,  giving 
a  branching  type  of  discharge  ;  and  many  interesting  irregu- 
larities of  direction,  color,  and  form  are  produced. 

The  light  from  the  flash  moves  with  great  velocity  while 
the  sound  of  the  thunder  travels  slowly,  at  the  rate  of 
ordinary  sound  waves.  The  sound  wave  is  readily  worn 
out,  and  at  a  distance  of  a  few  miles  lightning  produces 
no  perceptible  sound.  Heat  lightning  is  often  the  result 
of  the  reflection  among  the  clouds,  or  on  the  horizon,  of 
lightning  in  some  far-distant  thunderstorm,  perhaps  en- 
tirely hidden  behind  the  curvature  of  the  earth. 

Heat.-^ — Aside  from  the  heat  which  comes  to  us  from  the 
sun,  we  obtain  a  certain  small  but  more  constant  supply  from 
the  other  bodies  of  space  and  from  the  earth  itseK;  but 
these  are  relatively  unimportant.  The  radiant  energy  from 
the  sun  travels  at  an  enormous  velocitv  as  a  series  of  waves, 
wliich  are  radiated  out  from  the  sun  in  all  directions ;  and 
only  that  small  portion  of  them  is  received  by  the  earth 
which  it  happens  to  intercept  in  its  passage  about  the  sun. 

Some  substances  allow  this  energy  to  pass  through  them 
with  readiness,  and  these  are  said  to  be  diathermanous ; 
others  absorb  it ;  and  still  others  reflect  the  greater  part 
of  the  rays  that  come  to  them.  The  air  is  comparatively 
diathermanous,  as  indeed  most  transparent  substances  are. 
The  smooth  glassy  surface  of  water  is  a  good  illustration 
of  a  substance  that  reflects  much  of  the  radiant  energy 
coming  to  it.  On  the  other  hand,  while  the  earth  reflects 
some,   it   absorbs   a   large   quantity   of    heat ;    and   this   is 

1  The  sun  is  emitting  a  form  of  energy  which  under  favorable  conditions  be- 
comes heat,  while  under  other  conditions  it  takes  the  form  of  chemical  energy 
riiese  rays  are  therefore  properly  radiant  energy  until  transformed  to  heat. 


THE  ATMOSPHERE.  31 

particularly  true   for   parts    of   the   earth  which   are   dark 
in  color. 

The  rays  that  enter  the  atmosphere  pass  through  it  with 
little  interference,  because  it  is  diathermanous  ;  but  if  there 
is  much  dust  or  water  vapor  in  it,  a  considerable  share  of  the 
rays  are  intercepted.  Thus  clouds  effectually  check  the 
passage  of  many  of  the  rays,  and  hence  cloudy  summer  days 
are  cool.  The  same  effect  is  produced  by  a  very  hazy  atmos- 
phere, and  in  the  late  afternoon  when  the  solar  rays  pass 
through  a  great  thickness  of  air  (Fig.  16),  the  amount  of 
heat  that  reaches  the  earth  is  very  much  less  than  that 
which  comes  to  the  surface  at  midday. 

Since  different  parts  of  the  earth's  surface  behave  dif- 
ferently toward  the  radiant  energy,  there  is  much  varia- 
tion in  the  effect  produced.  This  is  particularly  well 
illustrated  by  the  very  marked  difference  in  behavior  be- 
tween water  and  land.  The  rays  that  reach  the  water  sur- 
face are  in  part  reflected  back  into  space  and  thus  lost,  so 
far  as  the  earth  is  concerned.  Much  of  that  which  remains 
raises  the  temperature  of  the  water ;  but  as  the  specific 
heat  of  the  water  is  high,  its  temperature  is  raised  very 
slowly.  Some  is  used  in  the  evaporation  of  the  surface 
layers  ;  and  in  that  case  the  solar  rays  are  transformed  to 
the  so-called  "  latent  heat,"  ^  which  does  not  become  appar- 
ent until  the  vapor  is  condensed  to  water.  Moreover,  the 
water  surface  is  in  motion ;  and  this  tends  to  distribute  the 
heat,  and  thus  to  prevent  the  excessive  warming  of  the  ocean 
surface.  Therefore  for  these  various  reasons,  even  at  the 
equator  the  ocean  surface  remains  relatively  cool. 

On  the  other  hand,  land  reflects  very  little  of  the  radiant 
energy,  and  it  is  a  solid  bod}^,  in  which  neither  evaporation 

1  The  old. term  is  still  used,  though  perhaps  heat  of  vaporization  would 
be  better. 


32  PHYSICAL   GEOGRAPHY, 

nor  motion  is  possible.  The  earth  is  distinctly  not  diather- 
manous,  and  the  greater  part  of  the  rays  which  reach  it  are 
absorbed  by  the  surface  portions.  Therefore  during  the 
day  the  ground  tends  to  become  warmed  by  absorption  ; 
and  this  peculiarity  is  responsible  for  many  of  the  phenomena 
of  the  atmosphere,  which  are  later  described. 

Pure  air  is  very  slightly  warmed  by  the  passage  of  the 
direct  rays  of  the  sun.  The  small  amount  of  heat  thus 
obtained  is  slightly  increased  by  a  supply  received  from  the 
rays  which  the  earth  reflects ;  but  much  more  is  obtained 
from  the  supply  which  the  earth  absorbs.  All  bodies  in 
space  are  radiating  a  form  of  energy,  either  that  which 
belongs  to  them  or  that  which  is  radiated  to  them ;  there- 
fore the  earth  is  at  all  times  emitting  rays  by  direct  radi- 
ation. During  the  daytime  the  amount  radiated  is  less 
in  quantity  than  that  received  from  the  sun  ;  but  at  night, 
when  this  supply  is  cut  off,  the  process  of  radiation  proceeds 
so  far  that  the  earth  loses  much  of  the  heat  which  it  had 
received.  Radiation  is  interfered  with  by  the  presence 
of  clouds  or  dust;  and  hence  nights  which  are  cloudy  or 
hazy  are  warmer  than  those  which  are  clear. 

By  the  process  of  conduction,  all  bodies  which  are  warmed 
tend  to  transmit  their  energy  to  cooler  portions.  This  is 
well  illustrated  when  a  cold  iron  is  placed  upon  a  w^arm 
stove.  In  the  same  way,  the  air  in  contact  with  the  warmer 
earth  is  thus  warmed  by  conduction  ;  but  neither  air  nor 
earth  are  good  conductors  of  heat,  and  if  this  process  were 
unaided,  the  effect  would  be  slight  and  confined  to  those 
lower  layers  of  the  air  which  were  almost  immediately  in 
contact  with  the  earth.  It  is  a  property  of  gases  that  when 
heated  they  are  expanded  and  thus  made  lighter.  By  this 
means  a  process  of  convection  is  started  which  bears  some 
analogy  to  the  boiling  of  water,  and  the  warm  lower  layers 


THE  ATMOSPHERE.  33 

of  air  rise  above  the  surface,  because  the  colder  and  denser 
air  forces  the  lighter  layers  to  ascend. 

The  process  of  convection  is  one  of  the  most  important 
in  meteorology ;  for  upon  it  in  large  measure  depends  the 
development  of  the  winds  and  other  features  of  atmospheric 
circulation.  When  air  rises  it  expands,  and  in  the  process 
of  expansion  necessarily  cools,  the  rate  of  cooling  being 
1.6°  for  every  300  feet  of  ascent;  and  descending  air,  as 
a  result  of  compression,  becomes  warmed.  This  feature  of 
cooling  on  ascension  gives  rise  to  the  formation  of  many 
of  the  clouds  and  rainstorms. 

Thus  the  air  is  warmed,  partly  by  the  rays  which  come 
direct  from  the  sun ;  partly  by  those  which  are  reflected 
from  the  earth ;  partly  by  those  emitted  from  the  earth 
by  the  process  of  radiation ;  but  mainly  by  conduction 
from  the  warm  earth's  surface  and  the  convectional  rising 
of  these  warmed  layers.  Highlands  are  cooler  than  low- 
lands, largely  because  the  air  in  these  places  is  less  dense 
than  that  nearer  the  sea  level  (Fig.  15).  The  presence  or 
absence  of  large  bodies  of  water  very  markedly  modifies 
the  effect  of  solar  energy  upon  the  atmosphere.  As  a 
result  of  these  differences,  the  atmosphere  is  put  in  motion, 
winds  are  produced,  clouds  are  formed,  storms  are  started, 
and  rains  are  caused. 

The  movements  of  the  earth  in  space  also  give  rise  to 
many  variations  in  heat  effect  and  atmospheric  phenomena. 
As  a  result  of  the  rotation  of  the  earth,  the  greater  part 
of  its  surface  is  lighted  and  warmed  during  a  part  of  every 
twenty-four  hours,  and  thus  we  have  day  and  night. 

A  second  important  movement  of  the  earth  is  that  of 
revolution,  which  causes  the  seasons  (Figs.  8  and  17). 
Since  the  pole  is  inclined  to  the  plane  of  revolution,  the 
sun  is  made  to  appear  to  migrate  in  the  heavens.     During 


34 


PHYSICAL   GEOGRAPHY, 


our  winter,  when  the  sun  is  vertical  over  that  part  of  the 
earth  which  lies  between  the  equator  and  the  tropic  ol 
Capricorn,  the  sun  rises  in  the  southern  part  of  the  heavens, 
and  passes  westward  without  rising  high  toward  the  zenith. 
Then  in  Arctic  latitudes,  the  sun  does  not  rise  above  the 
horizon;  and  therefore  in  this  region  there  is  no  alterna- 
tion of  day  and  night.     In  the  winter  season,  in  temperate 


Fig.  17. 

Diagram  to  show  the  inclination  of  the  sun's  rays  in  different  parts  of  the  earth 
during  the  various  seasons.  Upper  figure,  spring  and  autumn ;  right-hand 
figure,  northern  winter ;  left-hand,  northern  summer. 


latitudes  the  journey  of  the  sun  across  the  heavens  occupies 
a  small  fraction  of  the  whole  day;  and  therefore  in  such 
regions  the  time  during  which  the  earth  is  receiving  heat 
is  less  than  the  length  of  the  night,  during  which  almost 
none  is  received. 

Besides   this   fact   of    short   days   and    long    nights,   the 
angle    at    which    the    rays    reach    the    surface    is    much 


TBE  ATMOSPEEBE.  35 

• 

more  oblique  than  in  the  summer  season ;  and  before  reach- 
ing the  surface  they  are  obliged  to  pass  through  a  great 
thickness  of  atmosphere.  These  facts  make  the  effect  of 
the  small  amount  of  energy  that  does  come,  less  apparent  in 
winter  than  in  summer,  when  many  of  the  rays  pass  from 
a  point  near  the  zenith  through  a  relatively  small  amount 
of  atmosphere,  reaching  the  surface  more  nearly  at  right 
angles  (Fig.  17).  After  the  sun  has  passed  north  of  the 
equator,  summer  comes  to  the  northern  hemisphere,  while 
winter  prevails  south  of  the  equator. 

Thus  at  any  point  between  equatorial  and  Arctic  regions, 
there  are  two  variations  in  the  effect  of  the  solar  rays,  one 
a  daily  and  the  other  a  seasonal  variation.  The  tempera- 
ture of  the  air  over  the  land  normally  rises  during  the 
day,  and  falls  at  night ;  it  rises  in  summer,  and  falls  in 
winter ;  and  the  amount  of  daily  rising  and  falling  is 
greater  in  summer  than  in  winter.  There  is  much  variation 
in  these  respects  according  to  latitude ;  and  there  is  less 
change  in  temperature  between  day  and  night,  and  between 
seasons,  at  the  equator  than  in  most  other  latitudes ;  but  the 
amount  of  heat  received  there  is  greater  than  in  other  parts 
of  the  earth.  The  greatest  range  in  temperature,  both  sea- 
sonal and  daily,  is  experienced  in  the  higher  latitudes.  The 
least  heat  supply  is  received  in  polar  latitudes ;  and  here 
there  is  a  great  range  between  the  summer  and  winter  tem- 
peratures, but  slight  daily  ranges,  because  in  winter  the  sun 
does  not  rise  above  the  horizon,  while  in  summer  it  does 
not  set. 

Moisture.  —  When  rays  of  radiant  energy  enter  a  water 
body,  they  are  in  part  transformed  to  "latent  heat,"  being 
engaged  in  the  process  of  changing  the  liquid  to  a  gaseous 
condition.  By  this  process  of  evaporation  much  of  the 
energy  exists  in  a  form  which  is  not  apparent  as  heat  so 


86  PHYSICAL   GEOGRAPHY. 

long  as  the  vapor  condition  lasts;  but  when  the  vapor  is  con- 
densed, this  store  of  heat  becomes  apparent.  Evaporation 
will  take  place  even  from  a  snow  surface ;  but  the  most 
favorable  conditions  for  the  production  of  water  vapor  are 
warm  air  in  contact  with  a  water  surface. 

The  capacity  of  the  air  for  water  vapor  is  limited ;  and 
when  no  more  can  be  contained  it  is  said  to  be  saturated. 
When  there  is  little  vapor  in  the  air  it  is  constantly  capable 
of  taking  more  until  the  limit  of  saturation  is  reached.  We 
commonly  say  that  dry  air  can  absorb  vapor. ^  If  the  amount 
of  water  upon  the  land  is  slight,  the  air  in  these  places 
remains  dry ;  but  naturally  this  cannot  be  the  case  with  air 
over  bodies  of  water,  for  there  the  conditions  favor  satu- 
ration. In  the  interior  of  continents,  and  in  the  upper 
layers  of  the  atmosphere,  there  is  the  smallest  proportion 
of  water  vapor.  If  the  air  from  these  places  reaches  the 
oceans,  it  may  bring  to  them  conditions  of  dryness,  which, 
however,  are  soon  changed  to  relative  dampness.  With  the 
air  in  movement,  saturxition  is  less  liable  to  occur  than 
would  be  the  case  if  the  air  were  quiet.  Therefore  winds 
favor  evaporation  by  bringing  fresh  supplies  of  air,  and  for 
the  same  reason  they  tend  to  prevent  saturation. 

The  capacity  of  air  for  water  vapor  also  depends  upon  its 
temperature.  A  layer  of  air  which  is  saturated  at  the  tem- 
perature of  50°  becomes  relatively  dry  if  its  temperature  is 
raised  to  90°;  and  an  air  layer  which  is  nearly  saturated  at 
90°  will  be  obliged  to  give  up  some  of  its  water  vapor  if  the 
temperature  is  lowered  a  number  of  degrees.  This  is  a  very 
important  point  in  the  formation  of  clouds,  storms,  and  rains. 
The  actual  amount  of  water  vapor  in  the  air  represents  its 

1  Strictly  the  air  does  not  absorb  vapor,  but  the  water  vaporizes  regardless 
of  the  presence  of  the  air.  However,  it  is  convenient  to  speak  of  the  capacity 
of  the  air  for  water  vapor,  especially  as  the  air  determines  the  temperature. 


TEE  ATMOSPHERE, 


3T 


absolute  humidity ;  but  this  is  not  a  very  important  factor, 
because  the  same  amount  of  vapor  in  air  of  different  tem- 
peratures will  produce  very  different  effects. 

The  point  of  greatest  importance  is  the  relative  humidity^ 
which  is  the  percentage  of  water  vapor  actually  contained  in 
the  air  compared  with  the  amount  which  the  air  at  that  tem- 
perature could  contain  if  it  were  saturated.  Thus  the  relative 
humidity  of  saturated  air  at  a  temperature  of  60°  is  100  per 


100 


80 


60 


40 


20 


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\  1 

J 

IJ 

w 

r 

\^       V      i 

1 

1 

1 

SEP.Il 


12 


13 


14 


Fig.  18. 


Diagram  showing  daily  change  in  relative  humidity  as  a  result  of  the  daily 

change  in  temperature  at  Ithaca,  N.Y. 


cent,  for  at  that  temperature  no  more  can  be  contained  ; 
but  if  the  temperature  is  raised  a  few  degrees,  the  air 
becomes  capable  of  containing  more  water  vapor,  and  the 
relative  humidity  is  then  less  than  100  per  cent.  The  tem- 
perature at  which  air  containing  a  given  amount  of  moisture 
becomes  saturated  is  known  as  the  dew  pointy  for  then  vapor 
must  be  condensed.  After  a  warm  and  apparently  dry  day, 
dew  may  be  formed  at  night  merely  by  lowering  the  tempera,- 


38 


PHYSICAL   GEOGBAPHY. 


1000  FT.  \  Saturated. 


500  FT 


ture  of  the  air,  and  thus  increasing  the  relative  humidity,  with- 
out any  change  whatsoever  in  the  absolute  humidity  (Fig.  18). 
It  follows  from  this  that  there  must  be  very  marked  differ- 
ences in  the  amount  and  effect  of  water  vapor  contained  in 
the  air.  Over  the  oceans,  the  relative  humidity  is  great,  and 
the  air  nearly  always  near  the  point  of  saturation ;  in  the 
tropics,  where  the  temperature  is  high,  the  absolute  humidity 
is  high,  because  warm,  air  can  contain  much  vapor ;  and  on 

mountain  peaks,  where  the  temperature  is 
low,  the  amount  of  vapor  is  slight,  because 
cold  air  has  little  capacity  for  water  vapor. 
If  the  dry  upper  air  descends  to  the  earth, 
its  absolute  humidity  is  low  ;  and  even  if  it 
commenced  its  descent  in  a  saturated  con- 
dition, its  relative  humidity  decreases  be- 
cause the  temperature  rises  (Fig.  19);  and 
if  air  currents  move  from  cooler  to  warmer 
latitudes,  their  capacity  for  vapor  is  con- 
stantly increasing,  because  they  grow  con- 
^  capacity  for  vapor  stautly  Warmer  and  have  a 
i,  fnlfmsed.^^  greater  power  of  absorbing 

vapor.      When   they   move 

Diagram  illustrating' increase  in  tempera-  ^^^m  warm  to  COOler  regions, 
tare  of  descending  air.  Starting  in  a  their  relative  humidity  in- 
saturated  condition  with  a  temperature  ,  i_i     •       -_ 

of  40°  at  1000  feet,  it  reaches  the  surface  creases,  because  their  tem- 

with  a  higher  temperature  and  its  ca-    perature       descends  ;       and 

pacity    for    vapor    increased,  while  its  .  . 

relative  humidity  has  decreased.    The    when    air     I'lSCS     OVer    land 

reverse  takes  place  with  ascent.  elevations,  or  vertically  by 

convection,  the  relative  humidity  is  also  increased,  because 
air  cools  by  expansion  as  it  ascends ;  and  under  such  condi- 
tions the  vapor  is  often  condensed  in  clouds  and  rain. 

As  a  result  of  these  varying  conditions  we  get  many  varia- 
ble phenomena.     Where  the  winds  are  prevailingly  dry,  and 


THE  ATMOSPHEBE.  39 

the  relative  humidity  low,  desert  conditions  result;  and 
where  moist  winds  rise  over  rapidly  ascending  lands,  condi- 
tions of  excessive  rainfall  are  produced.  With  air  prevail- 
ingly dry,  evaporation  is  rapid,  while  in  regions  of  great  rela- 
tive humidity,  evaporation  is  slow  and  small  in  amount  (Fig. 
60).  Since  water  vapor  contains  a  store  of  "latent  heat" 
great  stores  of  heat  energy  are  transported  from  one  latitude 
to  another  by  the  movements  of  vapor-laden  air  currents. 

Pressure.  — The  air,  though  so  light  and  apparently  almost 
without  substance,  actually  has  weight.  At  the  seashore, 
the  average  weight  of  the  air  column  is  15  pounds  to  the 
square  inch  ;  but  as  we  ascend  into  the  air,  whether  in  a 
balloon  or  on  a  mountain,  the  pressure  of  the  air  becomes 
less  and  less.  Aside  from  this  difference  in  air  pressure  the 
weight  of  the  column  of  atmosphere  at  any  single  point  is 
almost  constantly  changing.  This  is  due  to  the  fact  that  the 
air  is  very  elastic  and  is  subjected  to  a  complicated  series  of 
movements.  We  shall  be  better  able  to  understand  the 
causes  for  these  changes  in  pressure,  and  their  effects  upon 
the  atmosphere,  after  we  have  examined  in  more  detail  the 
subjects  of  air  temperatures  and  circulation. 

Effect  of  Gravity.  —  In  a  measure  heat  and  gravity  are  in 
conflict  in  their  effect  upon  the  air.  Heat  is  always  expand- 
ing, some  portions  more  than  others,  but  gravity  in  trying 
to  hold  the  air  to  the  earth  attracts  the  cooler  and  therefore 
denser  parts  more  strongly  than  it  does  the  lighter  warmed 
portions.  This  starts  a  movement  of  the  air,  for  the  denser 
portions  are  drawn  down  to  the  surface  and  the  lighter  parts 
pushed  above  it.  Gravity  is  thus  a  most  important  factor 
in  determining  the  equilibrium  of  the  atmosphere;  for  its 
constant  tendency  is  to  restore  an  equilibrium  which  other 
causes  are  tending  to  destroy. 

Effect  of  the  Earth^s  Rotation.  —  As  the  air  moves  in  the 
form  of  winds  or  currents,  there  is  a  constant  tendency  to 


40 


PHYSICAL   GEOGRAPHY, 


Fig.  20. 


be  deflected  to  one  side,  as  a  result  of  the  effect  of  the 
earth's  rotation.  This  not  only  tends  to  turn  the  currents 
of  air,  but  its  influence  is  also  felt  in  the  ocean  currents. 

In  the  southern  hemisphere 
the  currents  are  deflected 
toward  the  left,  and  in  the 
northern  hemisphere  toward 
the  right ;  and  we  common- 
ly speak  of  the  latter  as  the 
right-hand  deflection  (Fig. 
20). 

The  reason  for  this  deflec- 
tive tendency  is  to  be  found 
in  the  fact  that  different 
parts  of  the  earth  are  mov- 
ing  at   different  velocities. 

Diagram  to  show  how  the  moving  currents     gy  revolving  an  orange  Or  a 
are  deflected  from  a  straight  line  N-S.       ,     ,,  -,  . 

ball  around  an  axis  one  can 
see  that  the  motion  at  the  equator  is  much  more  rapid  than 
that  at  the  poles.  Each  revolution  carries  every  point  along 
a  circle,  but  the  diame- 
ter of  the  circle  de- 
creases toward  the  pole 
(Fig.  21).  Therefore 
in  the  course  of  a  revo- 
lution a  point  near  the 
equator  travels  a  much 
greater  distance  than 
one  near  the  pole.     To  p^^  21 

do  this,  it  must  go  faster.     Diagram  illustrating  the  decrease  in  diameter 

since  the  same  period  of  ^^  ^^^"'^"°*  i^tMnA^^. 

time  is  allowed.      At  the  equator  the  rate  is  1521  feet    a 

second,  while  near  the  poles  the  rate  is  greatly  reduced. 


THE  ATMOSFHEBE.  41 

A  current  moving  toward  the  equator,  from  a  region  of 
slow  motion,  is  constantly  reaching  latitudes  where  the  angu- 
lar velocity  is  greater.  If  the  earth  were  quiet,  it  would 
move  in  a  straight  line,  and  if  the  earth's  rotation  did  not 
produce  any  effect,  it  would  do  the  same  and  reach  a 
point  on  the  equator  toward  which  it  had  originally 
started  (N-S,  Fig.  20).  But  the  earth  is  rotating  toward 
the  east,  and  the  current  is  of  course  carried  along ; 
but  in  different  parts  of  its  course  it  is  carried  at  differ- 
ent rates.  There  are  therefore  two  motions,  one  to  the 
south,  the  other  to  the  east.  As  the  current  in  its  southerly 
course  reaches  regions  with  a  greater  velocity  than  those 
just  left,  it  lags  behind  the  earth's  rotation  just  a  very  little. 
In  other  words,  it  tends  to  take  to  regions  of  greater  velocity 
the  velocity  of  a  region  with  a  slower  motion.  This  lagging 
behind  turns  it  to  the  west,  or  the  right,  and  as  it  moves 
from  place  to  place  (Fig.  20)  it  keeps  turning  little  by  little, 
until  finally  its  course  is  very  much  altered.  Currents 
moving  northward  from  the  equator  pass  into  regions  of 
less  velocity  and  thus  run  ahead,  or  turn  to  the  east  in  the 
direction  of  the  earth's  rotation.  The  same  explanation 
holds  for  the  left-hand  deflection  south  of  the  equator.  ^ 

A  current  moving  very  slowly  will  so  nearly  accommodate 
itself  to  the  change  in  velocity  that  the  deflective  tendency 
is  not  very  effective.  Also  in  those  latitudes,  such  as  the 
equatorial  (Fig.  21),  where  the  difference  in  velocity  is  not 
great,  the  deflective  tendencj^  is  not  nearly  so  great  as  in 
the  higher  latitudes,  where  even  in  a  small  distance  there 
is  a  marked  difference  in  angular  velocity. 

Even  in  currents  moving  along  east  and  west  lines  the 

1  The  teacher  will  do  well  to  illustrate  this  important  point  by  the  use  of 
the  globe,  or  better  by  allowing  a  marble  to  run  over  the  face  of  a  rapidly 
revolving  wheel  which  is  inclined  toward  the  class. 


42  PHYSICAL   GEOGRAPHY. 

deflective  effect  is  apparent,  but  this  cannot  be  easily  ex- 
plained in  a  few  words. 


-•o*- 


REFERENCE    BOOKS. 

See  also  references  at  the  close  of  Chapters  III. -VII. 

Davis. — Elementary  Meteorology.  Ginn  &  Co.,  Boston,  1894.  8vo. 
.|2.70.  (Almost  all  points  thoroughly  treated  in  the  light  of  the  best 
modern  knowledge.) 

Loomis.  —  Treatise  on  Meteorology.  Harper  Brothers,  New  York,  1870. 
8vo.    $1.50. 

Scott.  —  Elementary  Meteorology.  Scribner,  New  York  (Agents) .  Fifth 
edition,  1890.     12mo.    $1.75. 

Tail.  — Light.  Macmillan  &  Co.,  New  York  (Agents).  Second  edition, 
1889.     8vo.     $2.00. 

Capron.  — Aurora.  E.  &  F.  N.  Spon,  New  York  (446  Brown  St.),  1879. 
4to.    $17.00. 

Guillemin  (translated  by  Thompson). — Electricity  and  Magnetism. 
Macmillan  and  Co.,  New  York,  1891.  8vo.  $8.00.  (Much  on  atmos- 
pheric and  terrestrial  electricity  and  magnetism.) 

Maxwell.  —  The  Theory  of  Heat.  Longmans,  Green,  &  Co.  Tenth  edi- 
tion.    (Edited  by  Lord  Ray leigh.)     1892.     12mo.     $1.50. 

Tyndall.  —  Heat  as  a  Mode  of  Motion.  Appleton  &  Co.,  New  York. 
Fourth  edition,  1883.     12mo.     $2.50. 

In  most  good  books  on  physics,  the  subjects  of  heat,  light,  and  electricity 
are  well  treated  from  the  physical  standpoint. 

The  American  Meteorological  Journal  (monthly,  Ginn  &  Co.,  Boston) 
contains  a  record  of  the  progress  in  the  subject,  and  many  original  articles 
of  general  interest.     $3.00  a  volume  ;  eleven  volumes  published. 


CHAPTER   III. 

DISTRIBUTION     OF    TEMPERATURE. 

General  Statement.  —  If  nothing  were  present  to  interfere 
with  or  to  distribute  the  solar  rays  that  come  to  us,  we 
should  have  a  very  regular  distribution  of  heat  over  the 
earth's  surface.  At  the  equator  the  temperature  would 
be  extremely  high,  much  higher  than  at  present ;  in  the 
Arctic  latitudes  there  would  be  very  low  temperatures  ;  and 
between  these  two  belts  there  would  be  intermediate  condi- 
tions. In  each  of  these  belts  there  would  be  seasons,  and 
the  difference  between  the  day  and  night  as  at  present. 
This  theoretical  distribution  of  the  solar  heat  is  in  reality  so 
well  defined  that  we  are  able  to  divide  the  earth's  surface 
into  three  great  climatic  zones, — the  Arctic,  Temperate, 
and  Tropical  belts  (Fig.  QB). 

In  each  of  these  zones  there  is  a  regular  normal  variation 
in  the  temperature  of  the  different  seasons,  there  being  a 
gradual  rise  from  winter  to  summer  which  with  the  corre- 
sponding descent  from  summer  to  winter  makes  what  we  may 
call  the  seasonal  range  or  curve  (Fig.  24).  By  the  rise  of  tem- 
perature during  the  day,  and  its  fall  at  night,  a  daily  curve 
is  also  produced  (Figs.  22,  27-29,  and  33)  ;  and  therefore  the 
seasonal  curve  is  made  up  of  a  large  number  of  daily  curves 
(Fig.  23).  Theoretically,  these  should  all  be  regular,  and 
season  after  season  we  should  have  an  almost  exact  repetition 
of  these  curves.     However,  in  reality,  this  is  far  from  being 

43 


44 


PHYSICAL   GEOGRAPHY. 


Fahr.  M 

100° 


19 


M 


80 


10^ 


60° 


60° 


'40° 


SO' 


to" 


10° 


-w° 


w 


the  case ;  and  the  divergence  from  the  theoretical  is  due 
to  the  presence  of  a  number  of  disturbing  influences.  These 
are  (1)  the  effect  of  atmospheric  movements,  (2)  the  in- 
fluence  of   the    oceans,   or   the   absence   of   such   influence, 

(3)  the  effect  of  topography. 
Effect  of  Atmospheric  Move- 
ments. —  This  subject  is 
again  referred  to  in  the 
chapter  on  winds,  and  now 
we  need  only  consider  a 
few  of  its  general  features. 
There  is  a  regular  circula- 
tion of  the  atmosphere,  and 
numerous  other  movements 
which  we  may  call  irregular. 
Certain  winds  blow  with 
moderate  steadiness  toward 
the  equator,  where  the  air 
rises  and  then  flows  away 
at  a  considerable  elevation 
above  the  earth's  surface. 
By  this  means  much  of  the 
heat  which  reaches  equatorial 
regions  is  borne  away  and 
^.  r  .  ^   .  ,        Lowest,  Arc-    (iistributed  in  other    zones. 

tic  (winter) ;    second,  north  temperate 

land   interior    (winter) ;    third,    same     In  the  north  temperate    lati- 

IroZTi;  Jprf  fiftr '^^''^  '''^"  '^^'    tudes  the  general  movement 

tropics  (winter) ;  fifth,  same  (summer) .  o 

of  the  atmosphere  is  toward 
the  east ;  and  this  brings  to  west  coasts  the  warm  air  from 
over  the  oceans,  while  to  the  eastern  parts  of  continents,  air 
is  brought  from  the  interior  regions.  By  means  of  these 
and  other  general  influences  of  the  atmospheric  circulation, 
the  temperature  of  the  earth's  surface  is  greatly  modified. 


/^ 

V 

^ 

\ 

^ 

'  ^^ 

vj 

^ 

t^ 

A 

V 

/ 

\ 

\^ 

J 

/ 

\ 

-^ 

r 

\ 

/ 

t 

\ 

/ 

\ 

/ 

\ 

V 

.J 

.^ 

*^****^ 

tTSS^PJ++ 

— 

trff*^ 

w 


w 


Fig.  22. 

Daily  temperature  curves 


DISTBIBUTION   OF  TEMPERATURE. 


45 


Smaller  movements  do  locally  what  these  great  movements 
do  in  a  general  way.  Tims  a  storm  passing  across  the 
country  brings  conditions  of  cloudiness  and  rain,  and  pro- 
duces winds  which  are  sometimes  warm  and  sometimes  cold. 
By  this  means  air  is  sometimes  drawn  from  cold,  snow- 
covered  lands ;  or  it  settles  from  the  upper  cold  layers  of 
air ;  or  it  may  be  drawn  from  the  equable  ocean.  At  the 
seashore,  during  the  summer,  the  cool  sea  breeze  may  blow 
and  modify  the  heat  of  the  hot  summer  day  (Fig.  38). 


Fig.  23. 

Diagram  illustrating  mean  seasonal  rise  in  temperature,  wltli  daily  and  irregular 

changes  superimposed. 


Influence  of  Oceans.  —  The  ocean,  and  even  large  bodies 
of  fresh  water,  are  important  modifiers  of  climate.  As  we 
have  already  seen,  the  ocean  water  warms  very  slowly,  and  it 
cools  with  almost  equal  slowness.  Therefore  the  difference 
between  the  temperature  of  day  and  night,  and  summer  and 
winter,  is  much  less  there  than  on  the  land,  which  warms 
rapidly  during  the  summer  day  and  cools  readily  at  night 
and  in  winter.      Over  the  ocean,  in  tropical  latitudes,  the 


46  PHYSICAL   GEOGRAPHY, 

temperature  range  throughout  the  year  is  very  slight ;  and 
in  temperate  latitudes,  while  the  range  is  much  greater  than 
this,  it  is  still  small  compared  with  the  range  on  the  land 
(Fig.  24).  Therefore  near  the  seashore,  the  temperatures  of 
the  summer  and  the  day  are  relatively  low,  while  the  tempera- 
tures of  winter  and  the  night  are  relatively  high.  Even  on 
the  shores  of  small  lakes  this  influence  of  water  is  noticeable. 

On  those  coasts  which  are  reached  by  prevailing  winds 
from  the  ocean,  as  on  the  west  coast  of  the  United  States, 
the  general  temperature  is  high,  and  the  climate  equable. 
Even  in  a  short  distance  the  temperature  difference  may 
be  very  marked ;  and  while  on  the  shore  the  effect  of  the 
ocean  is  plainly  felt,  this  influence  becomes  very  much 
less  marked  at  a  distance  of  a  few  miles  from  the  coast. 

Another  very  important  influence  of  the  ocean  is  that 
caused  by  the  fact  that  this  body  itself  is  in  motion.  Both 
warm  and  cold  ocean  currents  move  on  the  surface  of  the 
sea  and  tend  to  equalize  the  temperatures  of  different  parts 
of  the  earth.  By  this  circulation,  lands  that  would  other- 
wise be  uninhabitable  have  their  climate  rendered  much 
more  equable  than  that  of  regions  in  lower  latitudes  where 
these  conditions  of  oceanic  circulation  do  not  exist.  One 
of  the  best  illustrations  of  this  is  the  difference  between 
the  climate  of  Western  Europe  and  Eastern  America. 

As  a  general  statement  it  may  be  said,  that  under  the 
present  conditions  of  distribution  of  land  and  water,  ocean 
and  air  circulation,  and  alternation  of  day  and  season,  the 
general  climate  of  the  globe  becomes  progressively  colder 
as  the  polar  regions  are  approached ;  and  as  we  pass 
from  the  seashore  toward  the  interior  of  continents,  we  go 
from  regions  of  equable  climate,  to  those  possessing  great 
ranges  in  temperature  between  the  winter  and  summer, 
and  day  and  night. 


DISTRIBUTION  OF  TEMPEBATUBE.  4:1 

Effect  of  Topography.  —  It  would  be  quite  impossible  to 
enter  into  this  subject  in  much  detail.  In  general,  valleys 
are  warmer  than  hilltops,  partly  because  they  are  protected 
from  the  wind,  and  partly  because  the  solar  rays  that  fall 
upon  the  valley  sides  are  in  some  degree  reflected  into  the 
valley.  The  sides  of  hills,  or  of  mountains  which  face 
toward  the  sun,  are  warmer  than  the  north-facing  sides; 
and  this  is  often  very  well  shown  in  the  natural  distribution 
of  plants,  which  rise  higher  on  the  southern  side  of  the  hill 
than  on  the  northern  side,  where  the  temperature  is  less 
favorable  to  their  existence  (Fig.  68). 

Next  to  latitude,  altitude  is  probably  the  most  important 
feature  in  determining  climate.  If  the  elevation  be  sufli- 
cient,  conditions  in  some  respects  resembling  those  of  the 
Arctic  climate  may  be  found  even  under  the  equator.  At 
a  height  of  from  15,000  to  18,000  feet  above  sea  level, 
vegetation  ceases  to  exist,  and  perpetual  snow  covers  the 
mountain  tops.  This  is  due  to  several  causes,  the  most 
important  of  which  is  the  fact  that  the  air  at  great  eleva- 
tions is  less  dense  (Fig.  15),  and  hence  cooler.  Through 
this  relatively  thin  layer,  which  is  clear  and  free  from  large 
quantities  of  dust  particles  and  water  vapor,  the  rays  that 
fall  upon  the  surface  are  readily  radiated  into  space. 

This  illustration  is  interesting,  since  it  shows  that  in 
the  same  latitude,  and  consequently  with  the  same  amount 
of  solar  energy,  the  two  opposite  extremes  of  tropical  and 
Arctic  climates  may  result.  It  brings  out  very  strongly  the 
fact  that  the  mere  amount  of  energy  received  does  not 
determine  the  temperature  of  a  place ;  the  subsequent  be- 
havior of  this  is  equally  important.  This  same  fact  is 
shown  by  the  difference  between  the  climates  of  the  sea- 
shore and  the  land  at  different  places  in  the  same  latitude. 

Almost  everywhere  on  the  earth  the  influence  of  topog- 


48  PHYSICAL   GEOGRAPHY. 

raphy  upon  temperature  is  shown,  sometimes  in  great  differ- 
ences extending  over  wide  areas,  again  very  locally  and  in 
small  amount.  Mountain  ranges  prevent  tlie  passage  of 
vapor-laden  air  into  the  great  enclosed  basins,  where  dry 
clear  skies  exist,  and  where  desert  conditions  are  conse- 
quently produced  ;  and  we  might  find  many  instances,  great 
and  small,  to  illustrate  the  influence  of  land  forms  upon  the 
distribution  of  temperatures. ^ 

Seasonal  Temperature  Range.  —  From  the  above,  it  is  seen 
that  latitude  is  no  true  indication  of  temperature ;  for  it  is 
but  one  of  several  factors  which  tend  to  determine  climate. 
However,  it  is  one  of  the  most  important  of  the  factors,  and 
in  general  the  temperature  decreases  from  the  equator 
toward  the  poles.  Still,  owing  to  the  disturbing  influence 
of  the  other  factors,  this  decrease  is  not  regular  ;  and  hence 
the  lines  of  equal  temperature,  or  the  isotherms,  are  not 
parallel  to  the  lines  of  latitude,  but  often  diverge  very 
widely  from  them.  If  we  examine  the  charts  of  isotherms 
(Plates  2,  3,  and  4),  we  find  that  they  are  irregular,  and  that 
the  irregularities  vary  with  the  season.  Moreover,  any 
given  line,  such  for  instance  as  the  50°  isotherm,  is  in 
a  different  place  in  the  opposite  seasons.  In  other  words, 
the  temperature  of  every  part  of  the  earth  changes  with  the 
season ;  but  the  change  is  different  in  amount  in  different 
places. 

This  seasonal  change  may  be  called  the  temperature 
range  or  curve.  If  the  temperature  changes  of  any  given 
region  are  plotted  upon  a  diagram,  in  which  both  the 
months  and  the  scale  of  degrees  are  shown  (Fig.  24),  we 
find  that  there  is  a  gradual  rise  in  the  spring  to  a  time 
after   midsummer,  when   the   temperature   falls  until  after 

1  Many  of  these  features  are  illustrated  in  the  accompanying  isothermal 
charts. 


DISTRIBUTION   OF  TEMPERATURE. 


49 


midwinter.  Year  after  year  this  is  true,  though  each  year 
will  show  a  slight  difference  from  those  which  precede 
and  follow.  Even  in  different  regions  the  same  is  shown ; 
but  there  is  much  variation  in  the  form  of  the  seasonal 
curve  of  different  places.  Such  a  curve  shows  how  much 
difference  th^re  is  between  seasons,  and  when  it  occurs. 
We   find   that   the    height   to  wdiich  the   temperature  rises 


Jan. 


Feb.   March   April     May     June      July     Aug.     Sep.      Oct.      Nov.      Dec. 


70 


70 


Fig.  24. 
Seasonal  temperature  ranges.    Constructed  to  have  northern  and  southern  sum- 
mer coincide.    Hence  for  southern  hemisphere  June  should  read  January,  etc. 

in  the  curves  is  very  variable  in  different  parts  of  the  earth, 
and  the  same  is  true  of  the  length  of  the  warmer  or  the 
colder  part  of  the  curve,  which  is  the  same  as  saying  that 
the  length  of  the  warm  season  differs  in  different  places. 

If  we  plot  such  a  curve  as  this  for  a  place  over  the  ocean, 
we  find  that  it  is  relatively  flat,  because  the  difference 
between  the  winter  and  summer  temperatures  is  not  very 


60  PHYSICAL   GEOGBAPHY. 

great.  On  the  other  hand,  in  the  central  parts  of  continents, 
where  the  winter  is  relatively  cold,  and  the  summer  warm, 
the  curve  rises  to  a  much  greater  height.  At  the  equator, 
the  curve  is  much  flatter  than  in  temperate  and  Arctic 
latitudes,  where  the  difference  between  summer  and  winter 
temperatures  is  great.  In  any  one  of  these  zones  there 
may  be  marked  differences  even  in  neighboring  places. 

Upon  examining  one  of  these  seasonal  curves,  it  will  be 
noticed  that  the  time  when  the  temperature  is  highest  does 
not  correspond  with  the  period  when  the  greatest  amount 
of  heat  is  received  from  the  sun ;  nor  is  the  coldest  time  of 
winter  coincident  with  the  shortest  days.  In  other  words, 
there  is  a  lagging,  and  this  is  due  to  the  cumulative  effect 
of  the  heat  or  cold.  In  the  early  summer,  the  ground 
is  still  cool  from  the  effects  of  the  last  winter,  and  in  high 
latitudes  there  is  still  snow  upon  the  ground.  It  takes  some 
time  for  the  sun's  rays  to  warm  the  ground  and  the  air ; 
and  when  this  is  done,  the  effect  of  solar  energy  becomes 
greater  than  before,  even  though  the  days  be  shorter  and 
the  amount  of  energy  coming  from  the  sun  less  than  in  mid- 
summer. In  the  opposite  season,  the  effect  of  radiation 
during  the  long  nights  becomes  most  marked  after  the 
middle  of  winter,  which  is  really  the  22d  day  of  December. 
Therefore  January  is  almost  invariably  colder  than  Decem- 
ber, and  February  also  may  be  colder  than  December. 

For  the  sake  of  diagrammatic  illustration,  the  seasonal  curve 
is  represented  as  being  a  continual  rise  and  fall  of  tempera- 
ture. It  represents  the  average  temperatures  for  the  several 
parts  of  the  different  months.  In  reality  there  is  no  such 
regular  and  uniform  rise,  but  it  is  interrupted  by  daily 
risings  and  fallings  (the  daily  curve,  pp.  60-62),  and  by 
irregular  interruptions  (Fig.  23).  For  days  at  a  time  the 
normal  seasonal  rise  or  fall  may  be  interrupted,  and  even  be 


Face  pago  50. 


Isothermal  ( 


40 

POUNTNEY  &  CAKHlCHlEt.    ENSRS      BOSTON. 


tho  year. 


mSTJRIBUTION   OF  TEMPERATURE, 


51 


replaced  by  a  temporary  descent  (Fig.  25).  This  happens  in 
our  latitude  when  storms  or  cold  waves  pass  over  us,  and  pre- 
vent the  effect  of  the  sun's  heat  from  becoming  apparent. 
Thus  in  winter  we  may  have  thaws,  or  in  midsummer  the  heat 
may  be  tempered  by  several  days  of  cool  weather ;  but  there 
are  more  irregularities  during  our  winter  than  during  the 
summer.  The  temperature  curve  shows  only  the  average 
of  these,  its  chief  value  being  to  illustrate  the  effect  of 
the  sun's  rays  as  the  season  changes,  and  to  show  how  differ- 
ently this  effect  is  manifested  in  various  places. 


DAILY  MEAN  TEMPERATURES  OF  THE  STATE  FOR  1891,  WITH  NORMAL  VALUES 


JAMUAWT  rtSRUAHT  .*«CM  APRIL  |  MAT  |  JUNC  |  jm.r  [        AUCUST  |    SE^TtMBCH)        OCTQBtB    |       HQvtMeER    |      OECtMB 


»        I»       1»        «        U       »4 


i      tt     n    i  ♦       u     24    j  4        u     24     1 3      13     »3       I     7       17      «r       7       17     til     <       16      2ft  I    e       IB     M 


Fig.  25. 

Seasonal  curve  for  New  York  state.    Irregular  variations  shown  by 

the  lighter  line. 

Isothermal  Charts.  —  The  best  graphic  way  to  show  the 
distribution  of  temperature  over  the  earth,  is  by  means  of 
isothermal  charts.  The  isotherm  is  the  line  of  equal  tem- 
perature ;  and  the  chart  may  show  these  lines  for  the  day, 
or  for  the  month,  or  for  the  year.  If  for  the  year,  they 
represent  the  average  of  all  the  temperatures  during  that 
time ;  or  if  for  the  month,  the  same  average  for  day  and 
night  throughout  the  month.  Every  place  which  has  the 
same  average  temperature  for  the  period  represented  on 
the  chart,  has  the  same  isothermal   line.      That  is,  if   the 


52 


PHYSICAL   GEOGRAPHY. 


average  temperature  for  a  given  month  is  50°  at  London, 
Boston,  Buffalo,  etc.,  the  50°  isotherm  for  that  month  is 
made  to  pass  through  each  of  these  places. 

On  the  isothermal  chart  which  shows  the  average  tem- 


FiG.  26. 

Isotherms  for  February,  1878-1887. 


perature  for  the  year  (Plate  2),  it  will  be  noticed  that  in 
general  the  temperature  decreases  from  the  equator  toward 
each  of  the  poles ;  but  in  each  hemisphere  there  are  numer- 
ous exceptions  (Fig.  26).      The   rate   of  decrease   is  very 


DISTBIBUTION   OF  TEMPERATURE,  63 

variable  in  different  latitudes.  While  there  is  a  general 
tendency  for  the  lines  of  equal  temperature  to  run  parallel 
with  the  lines  of  latitude,  at  times  the  divergence  is  so 
great  that  the  isotherms  extend  in  a  north  and  south 
direction.  There  is  much  less  irregularity  in  this  respect 
in  the  southern  than  in  the  northern  hemisphere ;  and  this 
is  easily  explained  by  the  fact  that  the  land  is  mostly  in  the 
northern  hemisphere.  One  is  able  to  see  the  disturbing 
influence  of  the  land  in  many  places. 

Another  effect  of  the  greater  abundance  of  land  in  the 
northern  hemisphere,  is  that  the  belt  of  greatest  heat,  or 
the  heat  equator,  is  north  of  the  true  geographic  equator. 
The  land  becomes  much  warmer  than  the  ocean,  and  hence 
the  highest  temperatures  are  found  in  the  interior  of  conti- 
nents. This  is  not  because  more  energy  is  received,  but 
because  the  amount  that  does  come  is  much  more  effective 
in  warming  the  land  and  the  air.  Since  radiation  proceeds 
more  readily  from  the  land  than  the  water,  the  average 
temperatures  in  northern  regions  are  lower  than  in  the 
southern  hemisphere.  Other  general  influences  are  notice- 
able upon  the  chart  of  annual  isotherms.  For  instance,  in 
the  northern  Atlantic,  where  the  warm  Gulf  Stream  extends 
toward  the  Arctic  circle,  the  isotherms  are  bent  northward ; 
and  along  the  eastern  coast  of  the  United  States,  where  the 
cold  Labrador  current  floAVS  close  to  the  continent,  and 
isothermal  lines  are  bent  southward. 

On  the  western  side  of  North  America,  the  influence  of 
the  prevailing  winds  is  well  shown  where  they  blow  from 
the  warm  Pacific  upon  the  coast.  This  is  particularly  well 
illustrated  on  the  isothermal  charts  of  the  United  States 
(Plate  3),  where  we  see  a  very  marked  difference  in  the 
temperature  of  the  east  and  west  coast.  Thus  there  is 
a  great  range  in  temperature  between  Key  West,  on  the 


CO    S 

^    O 
P4     Si 


Face  page  55, 


P 

Tsotherma 


for  July 


DISTRIBUTION   OF  TEMPERATURE.  55 

extreme  southern  end  of  Florida,  and  the  northern  part 
of  the  coast  of  Maine,  while  in  the  same  distance  on  the 
west  coast  the  temperature  differences  are  much  less.  From 
Key  West  to  Cape  Hatteras  the  influence  of  the  warm 
Gulf  Stream  is  felt,  while  on  the  New  England  coast 
the  temperatures  are  lowered  by  the  cold  Labrador  current ; 
but  on  the  Pacific  coast  the  influence  of  the  warm  ocean 
is  manifest  from  Southern  California  to  Washington.  A 
study  of  the  charts  will  show  many  other  variations  in  the 
isotherms. 

In  the  isothermal  charts  which  represent  the  typical  sum- 
mer and  winter  conditions,  similar  phenomena  are  noticed ; 
and  in  some  cases  they  are  more  strikingly  illustrated  than 
on  the  annual  chart.  The  heat  equator  of  July  (Plate  4) 
follows  the  sun  well  up  toward  the  tropic  of  Cancer,  but 
it  does  not  follow  the  sun  as  far  when  it  takes  its  southern 
journey  during  our  winter  ;  and  in  the  Atlantic,  where  there 
is  much  more  neighboring  land,  the  migration  of  the  heat 
equator  is  more  marked  than  in  the  broad  Pacific.  We  notice 
also  that  the  influence  of  the  Gulf  Stream  in  deflecting  the 
isotherms  is  more  important  in  January  than  in  July, 
when  the  neighboring  ocean  waters  are  themselves  warmed 
by  the  summer  sun. 

In  the  two  hemispheres  there  is  also  a  difference  in 
in  the  amount  of  migration  of  the  isotherms  for  the  lower 
temperatures.  In  the  southern  hemisphere  the  isotherm 
of  50°  in  July  barely  reaches  Africa  and  Australia,  and  its 
position  in  January  is  not  greatly  different  (Plate  5). 
This  shows  the  influence  of  the  prevailing  condition  of 
water  in  that  hemisphere ;  and  the  same  fact  explains  the 
general  parallelism  of  this  isotherm  with  the  lines  of  lati- 
tude ;  but  in  the  northern  hemisphere,  where  there  is 
more  land,  the  isotherm  of  freezing  in  July  is  in  the  Arctic 


66  PHYSICAL   GEOGRAPHY. 

circle,  while  in  January  it  extends  below  the  40th  parallel  in 
several  places.  The  isotherm  of  50°  migrates  from  northern 
Scandinavia,  Iceland,  and  Labrador,  in  July,  to  Spain  and 
the  Carolinas  in  January. 

In  the  higher  latitudes  of  the  northern  hemisphere,  the 
influence  of  the  land  is  shown  by  the  fact  that  in  January 
excessively  low  temperatures  occur  in  the  interior  of  conti- 
nents. Thus  so  far  as  we  know,  the  coldest  parts  of  the 
earth  are  in  these  continental  interiors,  such  as  Asia.  The 
winter  ''  cold  pole  "  of  the  world  is  not  found  high  up  in  the 
Arctic  latitudes,  but  in  central  Siberia  near  the  Arctic  circle 
(Plate  5).  This  is  due  to  the  fact  that  in  these  dry  land 
interiors,  radiation  causes  excessive  cold  during  the  long 
winter  night.  It  is  possible  that  when  the  Antarctic  conti- 
nent or  the  interior  of  Greenland  are  better  known,  we  may 
find  upon  these  snow-covered  lands  even  lower  winter  tem- 
peratures than  those  of  northern  central  Asia. 

On  the  January  and  July  charts  of  the  United  States 
(Plates  6  and  7),  we  find  the  greatest  difference  in  tem- 
perature in  the  dry  interior  regions  of  Dakota  and  Mon- 
tana, and  the  least  at  Key  West  and  on  the  southern  coast 
of  California,  where  the  equable  ocean  waters  prevent 
either  excessively  high  summer  temperatures,  or  excessive 
cold  in  the  winter.  Another  place  where  the  temperature 
of  the  United  States  is  subjected  to  a  great  range  in  the 
different  seasons,  is  in  the  desert  region  of  the  Great 
Basin.  Here  the  sun's  rays  of  the  summer  day  readily 
pass  through  the  dry,  cool  air  and  raise  the  temperature 
of  the  ground,  and  the  lower  air  layers,  to  a  very  high 
degree.  At  night  and  in  the  winter,  radiation  proceeds 
with  rapidity,  because  the  air  is  clear  and  offers  little  ob- 
struction to  the  passage  of  the  radiant  heat ;  and  therefore 
in   the  winter   nights   the   temperature   becomes  very  low. 


Face  page  56. 


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DISTRIBUTION    OF  TEMPEBATUBE. 


59 


The  influence  of  topography  is  also  well  shown  in  several  por- 
tions of  the  charts  for  the  United  States,  and  also  on  the  New 


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York  chart  (Plate  8),  where  the  isotherms  are  seen  to  extend 
up  the  valleys,  showing  that  they  are  warmer  than  the  hills. 


60 


PHYSICAL   GEOGBAPHY. 


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Daily  Temperature  Curve.  —  The  daily  curve  represents 
for  the  day  what  the  seasonal  curve  does  for  the  year.  It 
shows  the  rise  in  temperature  during  the  daytime  and  its 
fall  at  night  (Fig.  27).  Unless  interfered  with  by  some 
accidental  cause,  the  temperature  rises  from  sunrise  till 
early  afternoon,  and  then  descends  until  late  in  the  night. 

As  in  the  case  of  the  seasonal  curve, 
the  time  of  highest  temperature  is  not 
when  the  sun's  rays  are  strongest,  nor 
is  the  coldest  part  of  night  at  midnight. 
The  explanation  is  the  same,  the  heat  of 
the  sun  in  the  morning  being  partl}^  ex- 
pended in  warming  the  earth  which  was 
cooled  in  the  preceding  night ;  and  the 
temperature  at  night  time  continues  to 
descend  after  midnight,  because  the  radia- 
tion of  the  heat  that  came  during  the  day 
proceeds  uninterruptedly,  and  its  influence 
is  not  checked  until  the  sun  again  rises. 

There  is  much  variation  in  the  daily 
curve  in  different  latitudes  (Fig.  22),  and 
even  in  different  places  in  the  same 
latitude.  The  daily  change  in  tempera- 
ture is  relatively  slight  on  the  seashore, 
and  very  great  on  the  land ;  and  the  range  is  much 
greater  in  temperate  latitudes  than  in  the  tropics.  In  the 
Arctic  regions,  where  the  sun  is  above  the  horizon  in  the 
summer  and  below  it  in  the  winter,  the  daily  curve  is  of 
very  little  importance,  and  may  be  entirely  masked  by  acci- 
dental causes. 

Since  in  many  parts  of  the  earth  there  is  a  great  variation 
in  the  length  of  day  and  night  during  the  different  seasons, 
the  daily  temperature  curve  varies  with  the  season.      Thus 


90 


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60 


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Fig.  27. 

A  normal  daily  range 
for  summer  and  for 
winter  in  New  York. 


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DISTRIBUTION   OF  TEMPERATUBE, 


61 


in  our  latitude  the  temperature  rises  much  higher  in  summer 
than  in  winter  (Fig.  27). 

While  normally  the  temperature  curve  is  that  which  has  just 
been  described,  in  reality  it  is  subjected  to  many  variations 
and  interruptions  (Fig.  28).     The  tendency  is  for  the  tem- 


MAY  8  9 


10 


11 


12 


13  14  15  16 

TEMPERATURE 

ITHACA 


17 


18 


20        MAY  21, 
1893 


Fig.  28. 
Normal  daily  curve  followed  by  an  interruption  of  several  days. 

perature  of  the  day  to  rise  above  the  average  for  that  season, 
and  to  fall  below  it  at  night  (Fig.  23).  Oftentimes  the 
daily  curve  is  so  changed  (Fig.  29)  that  instead  of  a  rise 
during  the  daytime,  we  have  a  fall  in  the  temperature 
(Fig.  64)  ;  or  the  temperature  may  continue  to  rise  through- 


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Fig.  29. 

Daily  temperature  record,  showing  interference  with  the  normal  rise 

and  fall  of  temperature. 

out  the  night,  the  opposite  of  what  would  normally  be  the 
case.  Cold  waves  or  storms  are  often  the  causes  for  these 
changes,  and  many  local  and  temporary  effects  may  thus  be 
produced.  The  presence  of  clouds,  or  of  much  moisture 
in  the  air,  or  of  winds  from  the  ocean  (Fig.  39),  may  very 


62 


PHYSICAL   GEOGRAPHY. 


effectually  modify  the  normal  daily  rise  and  fall  of  tempera- 
ture. 

Temperature  Ranges.  —  The  study  of  the  isotherms  of 
a  region  gives  us  an  idea  only  of  the  average  temperatures 
of  different  places.  In  a  study  of  climate  it  is  necessary 
to  know  something  of  the  changes  in  temperature,  both  with 
reference  to  the  amount  (Fig.  30)  and  the  rate. 


Fig.  30. 
Temperature  ranges  in  the  United  States  in  degrees  Fahrenheit,  1892. 


No  better  illustration  can  be  found  of  the  differences  that 
may  exist  between  places  on  the  same  isotherm,  than  that 
of  St.  Louis  and  San  Francisco,  which  are  on  the  same  annual 
isotherm  (55.7°)  and  on  nearly  the  same  parallel  of  latitude. 
In  San  Francisco,  the  average  for  September,  the  warmest 
month,  is  a  little  less  than  60°,  while  the  January  isotherm 
is  about  50°,  the  actual  range  between  the  averages  being 
about  9.5°.     At  St.  Louis,  the  January  isotherm  is  31°,  while 


DISTRIBUTION   OF  TEMPERATURE. 


63 


the  July  isotherm  is  78°,  a  range  of  about  47°.  Taking  the 
highest  and  lowest  temperatures  for  each  place,  the  differ- 
ence is  even  more  striking,  for  we  find  a  range  of  61°  in 
San  Francisco,  while  in  St.  Louis  the  range  is  128°.  There- 
fore, though  they  are  on  the  same  isotherm,  the  climates  of 
the  two  places  are  quite  different. 

The  lowest  temperature  ever  accurately  observed  on  the 
earth  was  less  than  —90°,  the  highest  over  127°,  the  former 


Fig.  31. 
Miniiuum  temperatures  observed  iu  the  United  States,  1892. 


in  Siberia,  the  latter  in  Algeria.  This  is  a  range  of  nearly 
218°.  Such  extreme  ranges  are  of  course  impossible  in  any 
single  place ;  but  in  some  of  the  dry  interiors  of  continents, 
very  extreme  temperature  ranges  are  sometimes  experienced. 
In  Siberia,  where  the  greatest  ranges  are  found,  temperatures 
181°  apart  have  been  observed ;  and  in  the  northwestern 
states  of  this  country,  ranges  of  over  150°  have  been  meas- 
ured.    On  the  other  extreme,  ranges  of  only  40°  or  less  are 


64 


PHYSICAL   GEOGBAPHT. 


observed  at  Key  West  and  on  the  coast  of  California.  In 
some  of  the  tropical  islands  of  the  Pacific,  the  greatest  differ- 
ence in  temperature  during  the  year  is  often  not  over  18° 
or  20°.  More  than  half  of  our  country  experiences  ranges 
greater  than  100°  (Figs.  30-32). 

If  these  temperature  changes  came  slowly,  their  effect 
would  not  be  so  very  difficult  to  endure ;  but  in  places  of 
great   annual  change,  there  is  almost  always  great  change 


Fig.  32. 
Maximum  temperatures  observed  in  the  United  States,  1892. 

in  short  periods.  In  Montana  (in  December,  1880)  in  less 
than  eighteen  days,  the  temperature  fell  117°,  the  thermome- 
ter on  the  12th  registering  58°,  and  on  the  29th  —59°.  In 
the  greater  part  of  northern  United  States  we  are  accustomed 
to  similar  changes  in  winter,  though  they  are  very  rarely  so 
extreme  as  this.  After  a  few  days  of  moderate  warmth 
during  the  unseasonable  winter  thaws,  a  cold  wave  spreads 
over  the  eastern  states,  and  zero  weather  prevails,  not  un- 


DISTRIBUTION   OF  TEMPERATURE.  65 

commonly  causing  a  drop  in  temperature  of  60°  or  70°  in 
a  few  days. 

We  are  even  liable  to  very  excessive  changes  in  a  single 
day.  Where  the  air  is  dry,  as  in  parts  of  the  arid  regions, 
a  change  of  40°  is  not  uncommon  in  the  summer,  as  the 
result  of  the  heat  of  the  day,  followed  by  the  coolness  of 
the  night,  which  is  caused  by  the  radiation  through  the  clear 
dry  air.  Near  the  ocean  the  difference  of  the  day  and  night 
temperature  is  often  very  slight,  particularly  in  the  winter. 
At  Key  West  the  day  and  night  temperatures  differed  only 
about  7°  in  December,  1877. 

Aside  from  these  regular  daily  ranges  there  often  occur  ex- 
ceptional changes  (Fig.  64).  In  winter  a  cold  wind  may  fol- 
low a  rain  storm  and  cause  the  temperature  to  descend  beloAV 
zero  with  a  change  of  35°  or  40°  in  a  few  hours.  In  this 
case  the  nocturnal  radiation  is  an  aid  in  the  fall  of  tempera- 
ture. A  daily  change  of  50°  is  not  uncommon  in  Montana ; 
and  in  Texas  the  thermometer  has  been  known  to  fall  63°  in 
sixteen  hours.  It  is  said  that  in  Thibet  the  temperature 
lias  fallen  90°  in  fifteen  hours,  or  from  68°  in  midday  to 
—  22°  at  night.  In  summer  there  are  also  great  ranges ;  but 
they  are  not  so  noticeable,  nor  are  they  so  severe,  as  those 
which  come  at  times  in  winter. 

Earth  Temperatures.  —  At  the  very  surface  of  the  earth 
the  ground  is  warmed  when  the  sun's  rays  are  present,  and 
cooled  when  their  effect  is  absent.  Below  a  depth  of  a  few 
feet  the  influence  of  the  sun  is  not  very  noticeable,  and  from 
this  point  downward,  the  temperature  of  the  earth  is  practi- 
cally permanent,  and  is  determined  by  the  heat  of  the  interior. 

There  is  much  difference  in  the  effect  of  changes  in  tem- 
perature in  different  parts  of  the  earth.  At  the  equator  the 
ground  is  very  warm  at  the  surface,  and  there  is  a  slight  varia- 
tion throughout  the  year.     At  the  depth  of  five  or  six  feet 


66 


PHYSICAL   GEOGRAPHY, 


the  intensity  of  the  heat  has  decidedly  decreased,  and  soon  the 
zone  of  no  variation  is  reached.  In  temperate  latitudes, 
the  difference  between  summer  and  Avinter  temperatures  is 
so  great  that  the  surface  becomes  warm  in  summer,  and  in 
winter  cools  down  to  temperatures  lower  than  the  freezing- 
point.  In  the  winter,  in  such  regions,  frost  exists  in  the 
ground  often  to  a  depth  as  great  as  six  or  eight  feet.  In 
the  Arctic  regions,  where  the  sun's  rays  are  of  little  power, 
and  where  radiation  is  excessive,  the  ground  is  often  per- 
manently frozen  to  a  depth  of  several  hundred  feet.  During 
the  summer  the  surface   layers   lose   their  frost  and  thaw, 

and  plants  grow  over 
earth  which  is  per- 
manently frozen. 

The  ground  is  such 
a   poor  conductor  of 
heat    that    it     takes 
many  weeks  for  the 
effect  of  the  summer 
heat,    or   the    winter 
cold,  to    reach    to    a 
depth  of  ten   or   fif- 
teen feet.     Therefore 
at    such   depths    the 
seasonal    curve    lags 
behind    that    of    the 
air ;   and  at  the  same 
time  the  temperature 
range  is  less. 
At  the  very  surface,  the  earth  temperature  changes  more 
than  that  of  the  air  (Fig.  33).     This  is  because  the  earth 
readily  absorbs  heat  and  radiates  it  with  almost  equal  rapidity. 
For  this  reason  the  ground  at  midday  is  warmer  than  the  air, 


Fig.  33. 

Daily  range  at  the  surface  of  the  earth  (dotted 
line)  and  of  the  air  ten  feet  above  (heavy  line). 
Ithaca,  N.  Y.,  July,  1893. 


BISTBIBUTION   OF  TEMPERATUBE.  67 

while  at  night  time  its  temperature  is  lower  than  that  of  the 
air.  These  facts  of  earth  temperatures  are  important  in 
explaining  the  heating  or  the  cooling  of  the  lower  air  layers. 


-*<>•- 


REFERENCE     BOOKS. 

See  also  Buchan's  memoir  referred  to  at  the  end  of  the  next  chapter ;  and 
also  the  hooks  on  general  meteorology,  notably  those  by  Davis,  Scott,  Waldo, 
Greely,  Abercromby,  Blanford,  Woeikof ,  Hann.  and  Croll. 

The  Berghaus  Atlas,  volume  on  Meteorology  (Hann,  Atlas  der  Meteo- 
rologie.  Justus  Perthes,  Gotha,  Germany,  1887.  15  marks  i),  although  in 
German,  contains  many  charts  upon  temperature  distribution,  etc.,  w^hich 
vyill  prove  of  value  in  the  schools. 

The  Annual  Reports  of  the  Signal  Service,  and  novp^  of  the  Weather 
Bureau  of  Washington,  contain  much  information  relating  to  the  tempera- 
ture, wind,  rain,  etc. ,  of  the  country .2 

Hazen.  —  The  Climate  or  Chicago.  Bulletin  X,  U.  S.  Weather  Bureau, 
Washington,  1893.  (Describes  some  interesting  effects  of  the  lake  upon 
temperature.    The  other  bulletins  of  this  series  are  also  of  value.) 

1  Under  the  present  law  governing  the  importation  of  foreign  books  no  duty  is  charged  on 
those  in  other  languages  than  the  English.  Foreign  books  may  be  ordered  direct  from  the 
publishers,  or  through  some  New  York,  or  other  importers.  With  all  charges  added,  the  mark 
becomes  equal  to  about  $0.25,  the  franc  to  about  $0.21,  and  the  shilling  to  about  $0.25 ;  but  in 
the  last  case  a  duty  may  also  be  charged.  While  this  does  not  give  the  actual  price,  it  furnishes 
a  close  approximation. 

2  Where  no  price  is  given  for  government  publications  it  indicates  that  they  are  distributed 
free  of  cost ;  but  in  many  cases  all  of  the  copies  are  exhausted,  and  the  only  way  to  obtain  them 
is  from  the  large  city  second-hand  bookstores.  Sometimes  it  is  not  possible  to  obtain 
government  publications  without  the  aid  of  a  congressman ;  but  this  will  be  easily  obtained 
by  most  schools. 


CHAPTER   IV. 

GENERAL    CIRCULATION    OF    THE    ATMOSPHERE. 

General  Statement.  —  Since  the  air  is  very  elastic,  and  its 
condition  easily  changed  by  variations  in  temperature,  it  is 
readily  caused  to  move.  No  better  illustration  can  be  found 
of  the  mobility  of  the  air  under  these  circumstances,  than 
that  which  is  so  often  noticed  on  heated  deserts.  The  ground 
becomes  warmed,  the  air  is  heated  by  contact  with  it,  and 
this  causes  the  air  to  expand  and  become  lighter  so  that  a 
tendency  to  rise  by  convection  is  produced.  Soon  this  ten- 
dency becomes  so  strong  that  the  lower  air  must  move  up- 
ward, thus  starting  a  dust  whirl  on  the  desert.  The  move- 
ment thus  started  by  the  effort  of  the  denser  air  to  take  the 
place  of  the  warmed  layers  causes  very  violent,  though  very 
local,  winds.  In  a  room,  a  warm  stove,  lamp,  or  an  open- 
grate  fire,  causes  the  air  to  move,  and  starts  a  circulation 
which  is  often  very  noticeable. 

The  reverse  process  of  cooling  the  lower  air  layers  causes 
a  condensation  which  necessitates  a  settling  down  of  other 
air.  We  may  often  see  an  illustration  of  this  on  a  cold 
winter  night  when  the  air  is  quiet.  If  the  window  in  a 
warm  room  is  then  opened,  the  cold,  dense  outside  air  flows 
in,  producing  a  very  perceptible  current. 

If  in  place  of  these  local  illustrations,  we  substitute  large 
areas  of  the  earth's  surface,  we  find  an  explanation  of  many 
of  the  greater  features  of  the  atmospheric  circulation.  Over 
equatorial  regions,  the  air  is  constantly  being  warmed  during 
the  day,  and  therefore  expanded.      Accompanying  this  ex- 

68 


GENERAL   CIBCULATION   OF  THE  ATMOSPHERE.       69 

pansioii,  there  is  rising  caused  by  the  greater  density  of  the 
surrounding  air,  and  so  a  circulation  is  produced  which 
exerts  its  influence  over  a  very  large  area.  This  circulation 
consists  of  four  parts :  (1)  the  inflowing  surface  winds, 
(2)  the   uprising   currents,   (3)  outflowing  winds   at   high 

N 


Ferrel's  ideal  diagram  of  the  planetary  circulation.    Dotted  arrows  show  upper 

currents  of  air. 

elevations,  and  (4)  down-settling  air  at  some  distance  from 
the  equator.  There  are  other  features  of  this  great  circu- 
lation which  we  will  soon  consider.  Similar  winds  upon  a 
smaller  scale  are  produced  over  continents,  and  even  on  the 
land  along  the  seashore. 

When  warm  air  is  expanded  and  raised  it  pushes  away  the 
air  above  it,  the  barometric  pressure  is  decreased,  because  the 
air  column  is  lighter ;  and  when  the  air  is  cooled,  it  becomes 


70 


PHYSICAL   GEOGBAPHY. 


denser,  and  hence  the  barometer  registers  a  higher  pressure 
of  the  air.  Therefore  the  relation  between  air  pressure  and 
wind  is  very  intimate  ;  and  where,  for  any  reason,  low-pres- 
sure areas  exist,  winds  are  found  blowing  toward  them 
(Plate  9).  This  is  the  case  in  certain  areas  which  are 
permanently  warmer  than  the  surrounding  regions,  and  also 
in  those  disturbances  of  the  air  which  are  classed  as  storms. 
A  barometric  gradient  is  produced,  and  the  winds  move  as 
if  they  were  going  down  grade.  The  air  moves  away  from 
high  and  toward  low  pressure  areas. 

Classification  of  the  Winds,  — For  the  sake  of  simplicity  in 
the  consideration  of  the  movements  of  the  atmosphere,  it 
seems  well  to  adopt  some  classification  of  air  movements. 
The  one  here  proposed  is  a  logical  division;  but  other  classi- 
fications might  be  used,  the  only  object  of  such  a  division 
being  to  group  like  kinds. 


Planetary  or  Perma- 
nent. (Due  to  plan- 
etary causes  of  a 
permanent  nature.) 


Periodical.     (Due  to 
periodical  causes.) 


'  Trades. 

Anti-trades. 
^  Doldrums,  or  equatorial  calms. 

Horse  latitude  winds  and  calms. 

Prevailing  westerlies. 

J  Migrating  winds  and  calm  belts. 

\  Monsoons. 

r  Land  and  sea  breezes. 

\  Mountain  and  valley  breezes. 


Irregular.  (Due  to 
causes  apparently 
of  an  irregular  na- 
ture.) 


Seasonal  winds 


Diurnal  winds 


Eclipse  winds. 
Tidal  breezes. 


Storm  winds 


Accidental  winds 


'  Desert  whirlwinds. 
Cyclonic  winds. 
Anticyclonic  winds. 
Thunderstorm  winds. 
Tornado  winds. 
Landslip  blasts. 
Avalanche  blasts. 
Volcanic  winds. 
Waterfall  breezes. 


Face  page  70. 


Winds  and  isobj 


^ — 29.80 


^jr~^~29.60 


globe  for  January. 


GENERAL   CIRCULATION   OF  THE  ATMOSPHERE.       71 

Planetary  or  Permanent  Winds.  —  There  are  certain  winds 
whose  force  and  direction  depend  upon  the  fact  that  there  is 
a  variation  in  the  amount  of  heat  received  in  different  lati- 
tudes of  the  earth,  and  that  the  earth  is  rotating  about  its 
axis.  These  may  be  called  planetary/  winds,  because  they 
would  be  developed  upon  any  planetary  body  where  similar 
conditions  prevail.  Or  we  may  call  them  permanent,  because, 
compared  with  other  winds,  their  direction  and  force  are  prac- 
tically permanent.  In  some  places  they  are  greatly  modified 
by  other  causes;  but  they  are  so  strongly  developed  that  their 
influence  is  felt  all  over  the  earth.  They  are,  as  it  were, 
the  general  atmospheric  winds ;  and  together  they  form  the 
fundamental  circulation  of  the  atmosphere.  They  may  be 
described  under  several  headings. 

Trade  Winds.  —  Since  the  air  over  the  equatorial  regions  is 
warmed  more  than  that  on  any  other  part  of  the  earth's  sur- 
face, the  denser  air  moving  in  toward  the  warmer  region 
causes  currents.  The  equator  may  be  fairly  compared  with 
a  stove,  over  which  air  rises  by  convection,  and  toward  which 
currents  flow.  These  inmoving  winds  are  called  the  trades, 
because  they  blow  with  marked  permanency  and  steadiness  ; 
and  in  planning  their  journey,  whenever  possible,  vessels 
choose  a  course  which  will  allow  them  to  take  advantage  of 
the  trade  winds.  These  winds  move  toward  the  equator 
(see  Plates  9,  10,  and  11),  but  instead  of  blowing  directly 
toward  it,  they  are  deflected  by  the  effect  of  the  earth's 
rotation.  North  of  the  equator  their  direction  is  from  the 
northeast,  while  south  of  it  they  move  from  the  southeast. 
They  are  much  less  well  developed  over  the  land  than  over 
the  water ;  and  when  they  blow  from  the  water  to  the  land, 
they  are  often  deflected  because  of  its  influence.  Over  the 
land  they  may  even  be  destroyed. 

The  air  in  the   trade  winds    is   moving  from  colder  to 


72 


PHYSICAL   GEOGRAPHY. 


—<ilffr'^ 


Plate  10. 
General  circulation  of  the  Atlantic  for  July. 


GENERAL   CIRCULATION   OF  THE  ATMOSPHERE.       73 


Fl^ 


Plate  11. 
General  circulation  of  the  Atlantic  for  January. 


74  PHYSICAL   GEOGRAPHY. 

warmer  regions,  and  therefore  its  capacity  for  water  vapor  is 
constantly  increasing.  Therefore  they  are  drying  winds,  and 
when  they  blow  over  the  ocean,  evaporation  is  rapid,  while  on 
the  land,  where  water  vapor  is  not  readily  obtained,  they  pro- 
duce deserts  in  many  places.  Since  the  temperature  of  this 
air  is  high,  when  blowing  over  the  ocean  the  amount  of 
water  vapor  which  it  is  enabled  to  carry  is  very  great;  and 
much  rainfall  is  caused  if  the  air  is  made  to  rise,  as  is  the 
case  when  the  trades  blow  upon  rising  coasts. 

Doldrum  Belt. — Over  the  heat  equator,  the  air  in  this  great 
planetary  circulation  rises  by  convection ;  and  in  this  place  a 
condition  of  almost  permanent  calm  is  produced  (Plates  10 
and  11).  This  is  particularly  the  case  over  the  oceans,  but 
over  the  land  other  causes  may  interfere.  The  doldrum  belt 
is  situated  between  the  north  and  south  trade  winds,  and  it 
migrates  from  season  to  season  as  the  heat  equator  changes 
its  position.  Since  the  air  in  this  belt  is  warmed,  it 
contains  much  water  vapor,  and  it  is  a  very  rainy  belt 
because  this  humid  air  rises  by  convection,  and  cools 
dynamically  until  the  dew-point  is  reached.  Therefore, 
during  the  day  the  sky  almost  invariably  becomes  cloudy, 
and  rains  fall. 

Anti-trade  Winds. — The  inflowing  of  surface  air,  and  its 
uprising,  makes  necessary  an  outflow  of  air  at  a  higher  level. 
This  outflowing  air  moves  away  from  the  equator,  in  either 
direction,  and  produces  what  is  known  as  the  anti-trades. 
These  winds  are  not  felt  on  the  land,  excepting  on  those  rare 
peaks  which  rise  to  a  height  of  10,000  or  12,000  feet  above 
sea-level.  They  move  in  a  northeasterly  direction  in  the 
northern  hemisphere,  and  toward  the  southeast  in  the  south- 
ern, in  each  case  being  turned  from  a  true  north  or  south 
direction  by  the  deflective  influence  by  the  earth's  rotation. 
Their  permanency  is  shown  by  the  fact  that  the  upper  clouds 


GENERAL   CIBCULATION  OF  THE  ATMOSPHERE,       75 

move  in  these  directions.  This  upper  air  movement  con- 
tinues in  the  temperate  latitudes. 

Horse  Latitude  Winds. — After  traveling  for  a  certain  dis- 
tance from  the  equator,  the  air  commences  to  settle,  and  some 
of  it  reaches  the  earth's  surface  near  the  poleward  margins  of 
the  trade  wind  belts.  These  regions  of  settling  air  are  known 
as  the  horse  latitudes^  and  because  the  air  is  descending  in 
these  belts,  the  prevailing  condition  is  that  of  calms  or  light, 
variable  winds ;  but  these  calm  belts  are  not  so  pronounced 
as  those  of  the  doldrums  (Plates  10  and  11).  Over  the  land, 
the  horse  latitude  belt  is  not  so  distinctly  developed. 

Prevailing  Westerlies, —  A  part  of  the  upper  circulation  of 
the  anti-trades  continues  toward  the  poles;  and  because  the 
polar  regions  are  places  of  permanent  low  temperature,  there  is 
a  tendency  for  the  upper  air  to  move  toward  them.  These  air 
currents  are  deflected  to  the  right  in  the  northern,  and  to  the 
left  in  the  southern  hemisphere,  so  that  in  the  upper  latitudes 
there  is  a  whirl  of  air  known  as  the  circumpolar  whirl,,  which, 
in  both  hemispheres,  produces  a  condition  of  prevailing 
westerly  winds,  both  near  the  surface  (Plates  10  and  11)  and 
in  the  upper  layers  of  the  atmosphere.  These  whirls  produce 
a  condition  of  permanent  low  pressure  in  the  polar  regions; 
for  there  is  an  eddy  produced,  which  is  somewhat  analogous 
to  that  formed  by  the  escape  of  water  from  a  bath  tub. 

In  the  upper  air  the  east-moving  winds  are  remarkably 
permanent,  as  any  one  may  see  by  watching  the  movements 
of  the  upper  clouds.  At  the  surface,  the  tendency  toward 
the  development  of  east-moving  air  currents  is  greatly  inter- 
fered with  by  other  causes.  This  is  much  more  strikingly 
shown  in  the  northern  than  in  the  southern  hemisphere, 
where  land  is  less  abundant.  In  the  latter  hemisphere,  sail- 
ing vessels  may  go  around  the  earth  with  prevailing  fair 
winds  driving  them    onward  (Plate  9).     To  do  this,  they 


76  PHYSICAL   GEOGRAPHY. 

must  go  past  Cape  of  Good  Hope,  and  return  by  way  of 
Cape  Horn.  In  the  northern  hemisphere,  the  most  striking 
influence  of  the  prevailing  westerlies  upon  the  surface,  comes 
from  the  fact  that  they  determine  the  path  of  movement  of 
the  greater  number  of  our  storms. 

Periodical  Winds.  —  There  are  certain  changes  of  a  period- 
ical nature,  which  tend  to  start  the  air  in  motion  in  a  definite 
way ;  and  this  tendency  is  repeated  as  these  periods  return. 
The  most  important  of  these  changes  are  those  which  arise 
from  the  variation  in  supply  of  solar  energy  in  the  different 
seasons,  and  in  the  change  from  day  to  night.  The  periodi- 
cal winds  may  therefore  be  classed  as  seasonal  and  diurnal 
winds ;  and  in  the  group  may  also  be  included  two  minor 
classes  of  periodical  winds,  eclipse  and  tidal  breezes. 

Seasonal  Winds.  —  We  get  a  large  supply  of  heat  in  one 
season,  and  a  very  much  smaller  amount  in  the  opposite 
season ;  and  the  differences  in  seasons  are  very  much  greater 
far  from  the  equator  than  they  are  in  the  equatorial  belt. 
Therefore  the  seasonal  effect  upon  the  atmosphere,  is  less 
marked  in  the  equatorial  belt  than  elsewhere  on  the  earth's 
surface.  Still,  even  in  equatorial  regions,  as  the  season 
changes,  the  movement  of  the  sun  in  the  heavens  produces  a 
very  decided  effect  upon  the  atmospheric  circulation. 

Migrating  Wind  and  Calm  Belts.  —  The  wind  charts 
of  the  Atlantic  Ocean  (Plates  10  and  11)  show  that  there 
is  a  migration  of  the  belt  of  calms  as  the  season  changes. 
The  trade  wind  belts  also  move  northward  and  southward ; 
and  therefore  in  one  season  a  region  within  the  tropics 
may  experience  the  calms  of  the  doldrums,  while  in  the 
opposite  season,  the  dry  and  permanent  trade  winds  blow 
steadily  day  after  day.  In  the  same  way,  a  region  situ- 
ated near  the  northward  or  southward  margin  of  the  trade 
wind  belts,  may  in  one  season  feel  the  trade  winds,  while 


GENERAL   CIRCULATION  OF  THE  ATMOSPHERE.       77 


Fig.  35. 
Summer  monsoons  in  India. 


in  the  opposite  season  the  variable  winds  of  the  horse  lati- 
tudes prevail. 

Monsoon  Winds. — In  latitudes  outside  of  the  tropics,  where 
large  land  masses  exist,  very  interesting  winds  are  often  caused 
by  the  difference  in  temperature  between  the  land  and  water. 
During  the  summer  the  land 
areas  become  warmed,  and 
are  covered  by  an  area  of 
permanent  low  pressure,  to- 
ward which  air  currents  move 
from  all  directions  (Fig.  35) . 
The  air  rises  over  the  warm 
land,  and  its  place  is  taken  by 
air  from  the  relatively  cool 
oceans.  In  the  winter  sea- 
son, the  land  becomes  cooler 
than  the  water,  and  the  air  is  caused  to  settle  over  the  land 
and  to  move  out  from  these  areas  of  high  pressure  (Fig. 
36),  toward  the  then  relatively  warm  oceans  (Plate  9).    This 

class  of  wind,  which  is  very 
pronounced  in  Asia,  is  known 
as  the  monsoon  wind.  Similar 
winds  are  noticed  in  other 
continents,  and  we  now  know 
this  class  of  air  movements  as 
the  monsoons. 

In  Asia,  the  monsoon  winds 
blow  in  toward  the  central 
regions,  across  India,  China, 
and  other  countries.  During 
the  winter  they  blow  in  the  opposite  direction.  In  Australia 
and  other  continents,  the  monsoon  system  of  winds  is  very 
well  developed;    and   on   the   Spanish  peninsula,  the  same 


Fig.  36. 
Winter  monsoons  in  India. 


78 


PHYSICAL    GEOGRAPHY. 


tendency  toward  inflowing  summer  and  outflowing  winter 
winds  is  quite  pronounced. 

Even  Avliere  the  distinct  monsoon  condition  is  not  pro- 
duced, a  tendency  to  the  production  of  this  class  of  wind 
often  expresses  itself  in  the  disturbance  of  the  wind  direc- 
tion. This  is  very  well  illustrated  along  the  Texas  coast, 
where  the  summer  trade  winds  are  deflected  until  they 
blow  upon  the  land.      This  phenomenon  is  shown  on  Plates 

10  and  11 ;  and  on  these 
charts  it  will  also  be  noticed 
that  in  the  winter,  the  pre- 
vailing winds  of  the  coast 
of  northern  United  States 
are  from  the  land  toward 
the  ocean,  while  in  summer 
their  direction  is  much 
less  definite.  That  is  to 
say,  the  prevailing  wester- 
ly winds  are  strengthened 
in  winter  and  weakened 
in  summer  by  the  monsoon 
tendency,  which  in  sum- 
mer is  not  sufficiently 
powerful  to  entirely  invert 
the  prevailing  westerlies. 
In  cold  regions,  such  as  Greenland,  there  is  a  tendency 
toward  the  production  of  outflowing  winds  in  summer, 
because  the  snow-covered  land  is  colder  than  the  water. 
Another  continental  effect,  not,  however,  dependent  upon  the 
temperature  differences,  is  the  retardation  of  winds  in  their 
passage  over  the  land.  As  a  result  of  friction,  the  winds  of 
the  land  are  less  violent  than  tliose  of  the  water;  and  mountain 
ranges  may  effectually  check  the  winds  and  destroy  them. 


Fig.  37. 
The  sea  breeze. 


GENERAL   CIRCULATION  OF  THE  ATMOSPHERE.       79 


Diurnal  Winds :  Sea  and  Land  Bi'eezes.  —  Since  the  heat  of 
the  day  is  followed  by  the  coolness  of  the  night,  the  atmos- 
phere is  often  caused  to  move  locally.  During  the  summer 
this  is  particularly  well  shown  along  the  seashore,  when  the 
familiar  sea  and  land  breezes  are  often  produced  on  calm 
days  and  nights.  During  the  day  the  land  becomes  warm 
and  air  tends  to  flow  toward  the  warm  areas  from  the  cool 
sea,  thus  producing  a  very  refreshing  sea  breeze  (Fig.  37). 
This  breeze  begins  to  blow 
late  in  the  morning,  and 
continues  until  the  power 
of  the  sun  has  decidedly 
diminished.  It  does  not 
come  every  day,  but  only 
when  other  atmospheric 
disturbances  are  not  mark- 
edly developed.  It  causes 
a  peculiar  disturbance  of 
the  normal  daily  tempera- 
ture curve.  In  ordinary 
cases  the  highest  tempera- 
ture of  the  day  comes  in 
the  mid-afternoon ;  but 
when  the  sea  breeze  com- 
mences to  blow,  the  temperature  usually  falls,  so  that  the 
highest  point  reached  during  the  day  may  be  before  noon 
(Fig.  39).  The  sea  breeze  does  not  generally  extend  far 
inland ;  and  ordinarily  at  a  distance  of  ten  miles  from  the 
coast,  it  is  hardly  perceptible. 

At  night,  when  the  land  has  become  cooler  than  the  ocean, 
a  very  gentle  breeze  often  blows  out  upon  the  water  (Fig. 
38),  rarely  more  than  a  few  miles  from  the  shore.  This  is 
the  land  breeze ;  and  thus  it  will  be  seen  that  by  the  daily 


Fig.  38. 
The  land  breeze. 


80 


PHYSICAL   GEOGBAPHT. 


change  in  temperature  there  is  produced  a  local  circulation 
resembling  in  a  small  way  the  more  extensive  continental 
or  monsoon  circulation  which  depends  upon  seasonal  changes 
in  temperature. 

Where  prevailing  winds  blow  upon  the  coast,  as  is  often 
the  case  in  the  trade  wind  belt,  the  intensity  of  these  winds 
is  sometimes  considerably  increased  during  the  day,  by  the 

6  NOON  6  M    combination  of  the 

sea  breeze  and  the 
normal  wind. 
Even  on  the  land, 
where  no  tendency 
to  the  production 
of  the  sea  breeze 
is  present,  the 
change  in  tem- 
perature between 
day  and  night  pro- 
duces an  effect 
Pj     gy  upon    the    winds. 

Diagram  showing  normal  daily  curve  on  a  hot  summer     i  he     heat    ot     the 
day,  and  the  effect  produced  by  the  sea  breeze.  davtimp  causes  the 

air  to  rise,  and  freshens  the  winds  by  increasing  their 
strength.  Thus  during  a  day  which  is  calm  in  the  morn- 
ing, strong  winds  may  arise,  and  die  down  as  the  sun 
sets. 

Along  the  shores  of  large  lakes,  a  lake  breeze  analogous 
to  the  sea  breeze  may  arise  during  hot  summer  days.  This 
is  particularly  noticeable  along  the  shores  of  the  Great  Lakes 
of  North  America,  and  it  is  one  of  the  reasons  for  the  strong 
winds  which  prevail  in  such  lake  shore  cities  as  Chicago. 

Mountain  and  Valley  Breezes.  —  Where  the  topography  of 
a  country  is  very  irregular,  as  in  mountainous  regions,  the 


GENERAL   CIRCULATION  OF  THE  ATMOSPHERE.        81 


Fig.  40. 
Mountain  breeze. 


change  in  temperature  between  day  and  night  often  pro- 
duces a  set  of  winds  known  as  the  mountain  and  valley 
breezes.  During  the  nighttime,  the  air  near  the  surface 
becomes  cool  by  radiation,  and  it  therefore  becomes  more 
dense  and  contracts.  This  dense  air  slides  down  the  moun- 
tain sides  into  the  valleys,  down  which  it  flows,  often  with 
sufficient  velocity  to  cause  gales. 
Just  as  streams  gather  water,  and 
thus  constantly  increase  their  ve- 
locity by  additions  from  their  up- 
per branching  tributaries,  so  these 
down-moving  air  currents  become 
concentrated  near  the  outlets  of 
the  mountain  valleys.  Among  the 
valleys  in  the  Rocky  Mountains, 
during  the  calm  clear  days  of  sum- 
mer, when  no  other  influences  are 
present  to  disturb  the  tendency,  these,  mountain  breezes,  or 
we  might  say  mountain  gales,  are  of  nightly  occurrence. 

The  heating  of  the  mountain  sides  during  the  day  causes 
an  updraft  of  air,  which  is  the  valley  breeze.  This  is  much 
less  intense  than  the  mountain  wind,  partly  because  the  air 
is  obliged  to  ascend  against  the  action  of  gravity,  and  partly 
because  the  upflowing  air  is  not  concentrated  during  its 
ascent,  but  is  rather  disseminated  over  the  mountain  and 
valley  sides.  The  day  breeze  becomes  most  intense  in  the 
mid-afternoon ;  and  the  night  breeze  attains  its  maximum 
development  just  before  sunrise. 

This  type  of  wind  is  by  no  means  confined  to  mountains, 
but  is  very  well  developed  in  plateau  regions,  such  for 
instance  as  that  of  central  New  York,  near  Ithaca  (Fig.  40). 
This  plateau  is  dissected  by  numerous  deep  valleys  which 
converge  toward  the  broad  depression  occupied  by  Lake 
o 


82  PHYSICAL   GEOGBAPHY. 

Cayuga.  At  nighttime  the  air  flows  down  these  valleys, 
producing  perceptible  breezes.  Concentrated  in  the  larger 
valley  occupied  b}^  the  lake,  the  wind  often  develops  into  a 
strong  breeze  during  the  calm  summer  nights ;  and  the 
wind  lasts  until  eight  or  nine  o'clock  in  the  morning.  No 
breeze  of  the  valley  type  has  been  noticed  here.  It  is 
probable  that  in  the  other  similar  valleys  of  this  region  the 
same  breeze  is  produced ;  and  it  may  be  expected  in  almost 
any  place  where  the  land  is  deeply  cut  by  valleys. 

Eclipse  and  Tidal  Breezes. — These  are  practically  unim- 
portant. During  total  eclipses  of  the  sun,  breezes  have  been 
noticed  whose  origin  seems  to  be  due  to  this  unusual  inter- 
ference with  the  sun's  rays.  Where  tides  rise  to  a  great 
height,  as  for  instance  in  the  Bay  of  Fundy,  local  observers 
report  that  an  increase  in  the  wind  accompanies  the  rising 
tide.  Little  is  known  about  this  type  of  tidal  breeze,  and 
it  is  possible  that  the  breeze  is  due  to  other  causes. 

Irregular  Winds.  —  These  winds  are  irregular  in  direction 
and  intensity,  and  they  depend  upon  causes  which  do  not 
return  with  regularity.  In  these  respects  they  are  quite  dis- 
tinct from  the  permanent  planetary  winds,  and  from  the 
regularly  recurring  seasonal  and  daily  winds.  They  there- 
fore deserve  to  be  grouped  in  a  separate  class.  Storm  winds, 
the  most  important  of  this  group,  are  considered  in  Chapter 
V.  Their  important  influence  in  disturbing  the  planetary 
circulation  is  well  shown  on  Plates  9,  10,  and  11. 

Accidental  Winds. — These  winds  are  rare,  and  depend  upon 
some  accidental  cause  which  starts  the  air  in  motion.  Per- 
haps the  most  common  wind  of  this  class  is  the  landslip  or 
avalanche  blast.  In  mountains,  and  more  rarely  in  other 
places,  large  masses  of  earth  and  rock  are  sometimes  pre- 
cipitated for  a  considerable  distance  down  some  steep  slope. 
These  landslides,  or  avalanches,  displace  a  considerable  mass 


GENERAL   CIRCULATION   OF  THE  ATMOSPHERE.       83 

of  air,  and  form  exceedingly  violent  local  winds,  which  at 
a  distance  of  a  few  hundred  yards  have  in  some  cases  been 
known  to  overturn  trees  and  houses. 

During  volcanic  eruptions  of  a  violent  nature,  vast  quan- 
tities of  air  are  started  in  motion ;  but  the  effect  of  these 
volcanic  winds  is  not  usually  important  upon  the  surface, 
because  the  displaced  air  is  high  above  the  ground.  The 
waterfall  breeze  is  a  gentle  breath  of  air  extending  out  from 
the  base  of  a  waterfall. 

The  Nature  of  Winds.  —  The  wind  is  a  bodily  movement 
of  the  air,  but  it  is  not  necessarily  a  steady  movement. 
Every  one  has  noticed  that  the  wind  blows  in  gusts,  and  that 
now  it  is  strong,  and  again  very  light.  In  some  respects 
these  pulsations  are  like  waves ;  and  it  has  lately  been 
found  that  even  when  the  wind  appears  to  be  blowing 
steadily,  it  is  really  made  up  of  a  large  number  of  gusts  or 
pulsations,  which  can  be  detected  only  by  very  delicate 
instruments.  Even  during  strong  winds  there  may  be 
momentary  calms. 

Nor  are  the  winds  a  perfectly  horizontal  movement  of  the 
air.  As  the  air  moves  over  an  irregular  country,  it  is  in 
some  cases  deflected  upward,  and  in  other  cases  downward. 
Another  reason  for  the  introduction  of  a  vertical  element 
into  wind  movements,  is  the  fact  that  upper  air  is  sometimes 
settling,  while  in  other  cases,  as  a  result  of  convection,  the  air 
near  the  surface  is  ascending. 

These  irregularities  in  air  movement  make  the  wind  an 
exceedingly  complex  series  of  motions,  in  which  the  pre- 
dominating direction  is  horizontal,  but  in  which  also  there 
are  a  number  of  vertical  movements.  It  seems  very  proba- 
l)le  that  it  is  these  vertical  movements  which  birds  make  use 
of  in  soaring.  Such  birds  as  hawks  and  eagles  are  able  to 
float  about  in  the  air,  and  even  to  rise  apparently  without 


84  PHYSICAL   GEOGRAPHY, 

making  movements  with  their  wings.  There  may  be  an 
internal  work  of  the  wind  which  these  birds  have  found,  and 
made  use  of  in  their  flight,  sorting  out  those  movements 
which  they  need,  and  not  being  retarded  by  those  which  are 
opposed  to  their  motion.  It  has  been  suggested  by  Professor 
Langley  that  it  is  not  impossible  that  these  air  movements 
may  be  employed  in  aerial  navigation  by  man  himself. 


-•o«- 


KEFERENCE    BOOKS. 

Ferrel.  — A  Popular  Treatise  on  the  Winds.  Wiley  &  Sons,  New  York. 
Second  edition,  1890.  8vo.  ^4.00.  (In  part  a  republication  of  Recent 
Advances  in  Meteorology,  Report  of  U.  S.  Signal  Service  for  1885,  Part 
II.,  Washington.) 

Buchan. —  Report  on  Atmospheric  Circulation,  Challenger  Reports, 
Physics  and  Chemistry,  Volume  II.  Eyre  &  Spottiswoode,  London, 
England,  1889.  4to.  62s.  6d.  (Contains  a  remarkable  series  of  charts 
relating  to  temperature,  pressure,  and  atmospheric  circulation.) 

See  also  the  general  books  by  Davis,  Waldo,  Greely,  and  others,  referred  to 
at  the  end  of  the  other  chapters. 


CHAPTER    V. 


STORMS. 


Cyclonic  Storms. —  As  used  here,  a   storm   is   any  con- 
dition of  cloudiness  accompanied  by  rain.     On  coasts  that 


Fig.  41. 

Ideal  diagram  of  a  storm.  Large  arrow  shows  path  of  storm;  small  arrows, 
inblowing  winds ;  circles,  lines  of  barometric  pressure ;  and  shaded  areas,  dis- 
tribution and  intensity  of  rain. 

rise  in  the  paths  of  moist  winds,  clouds  and  rain  are  often 
caused  by  the  condensation  of  water  vapor,  which  results 

85 


86 


PHYSICAL   GEOGRAPHY. 


from  the  rising  of  the  air,  and  the  consequent  cooling  until 
the  dew-point  is  reached.  In  the  same  way,  air  that  rises  as 
a  result  of  convection  may  reach  the  dew-point,  thus  form- 
ing clouds  and  rain.  These  kinds  of  rainstorms  are  not  of 
particular  importance  in  northern  United  States,  and  there- 
fore need  not  be  considered  in  detail. 

These  causes  aid  in  the  formation  of  the  very  important 
group  of  storms  which  bring  the  greater  part  of  the  rain 
that  falls  in  the  northern  half  of  this  country.  To  these  the 
name  cyclonic  storms  may  be  given  ;  and  these  are  not  of 


BAROMETER 

ITHACA 


29 


28 


^^^ 

^ — 

\  ^ 

/ 

.^~' 

^ 

^^' 

^^— 

"^ 

^^"^     ^i 

=7" 

^ 

y^ 

-^ 

\— /- 

\   / 

• 

\  / 

\  / 

\j 

29 


Aug.23     24         25         26         27 


28         29 
1893 

Fig.  42. 


30 


31      Sep.] 


28 


Fall  of  the  barometer  during  the  passage  of  tlie  center  of  a  hurricane 

near  Ithaca,  New  York. 

importance  merely  in  this  part  of  the  United  States,  but  they 
are  developed  on  many  portions  of  the  earth's  surface.  We 
may  divide  them  into  two  groups,  the  tropical  cyclones,  or 
hurricanes,  and  the  temperate  latitude  cyclones. 

Hurricanes.  —  Description.  In  the  warm  Atlantic  tropical 
belt  north  of  the  equator,  violent  storms  begin  and  move 
toward  the  American  coast,  along  which  they  pass  in  their 
course,  which  is  then  usually  northeastward  across  the  At- 
lantic. These  are  the  typical  hurricanes;  and  in  the  North 
Pacific  similar  storms  occur,  which  are  there  known  as 
typhoons.      Storms   of   this   nature   are   also  found   in   the 


ST0BM8. 


87 


m 


HW\    N 


South  Pacific  and  Indian  oceans;    but  none  occur  in  the 

South  Atlantic,  and  none  appear  to  originate  on  the  land. 

They  are  typical  productions  of  the  tropics,  and  in  these 

regions   often   attain   extraordinary   violence ;    but   in   our 

latitude,  although  they  are  the  most  severe  storms  that  we 

experience,  they  have  lost  much  of  their  tropical  violence. 

As  one  of  these  hurricanes  or 

typhoons   approaches    a    place, 

the  sea  is  calm  and  glassy,  and 

the  air  quiet  and  sultry.     The 

pressure    decreases    (Fig.    42), 

and  wind  begins  to  blow  with 

increasing  violence,  while  clouds 

overspread  the  sky,  at  first  as 

a  thin  hazy  veil,  which  gradually 

changes  to  a  solid  mass  of  dark 

clouds   from  which   rain  falls. 

The  wind  increases  to  a  gale  and 

gradually   shifts    its    direction, 

while    at   the    same    time    the 

barometer   falls.     If  the   place 

of  observation  happens  to  be  in  „       „ 

1^1  -     -  Fig.  43. 

the  path  of   the  center  of  the  T^•  *     •    ^^    -  a     •       -a 

^  ^  Diagram  of  spirally  inflowing  winds 

storm,  as  this  is  neared  the  wind       of  the  hurricane,  together  with  the 
1  '  '    ^  J  1         path  pursued  by  the  storm.    Spiral 

decreases  m  violence   and    sud-       Lvements  greltly  exaggerated. 

denly  changes  to  a  calm,  while 

the  sky  overhead  becomes  clear.  This  is  known  as  the 
"  eye  of  the  storm."  As  the  storm  passes  onward,  the 
wind  begins  as  suddenly  as  it  ceased  ;  but  this  time  it  is 
from  the  opposite  quarter,  and  then,  in  reverse  order,  con- 
ditions are  experienced  which  resemble  those  noticed  as 
the  storm  approached. 

These  conditions  indicate  that  the  storm  is   a   mass   of 


dm  ---'         V  / 


88 


PHYSICAL   GEOGRAPHY. 


whirling  air,  toward  the  center  of  which  the  winds  are  blow- 
ing from  all  directions,  along  spiral  courses,  as  is  shown  in 
the  accompanying  diagrams  (Figs.  41,  43,  and  44).  The 
hurricane  is  therefore  not  unlike  the  desert  dust  whirl  which 
was  described  in  the  last  chapter.  Air  is  moving  toward  a 
central  area,  where  it  ascends  and  flows  away  in  the    air 

above.  This  outflowing  of 
the  air  in  the  upper  parts  of 
the  storm,  is  shown  to  exist 
by  the  movements  of  the 
upper  clouds,  which  extend 
outward  as  long  streamers. 

Effects.  —  The  violence  of 
the  winds  in  a  hurricane  are 
almost  incredible,  and  many 
a  ship  that  has  been  drawn 
into  the  dangerous  whirl  has 
not  been  able  to  escape  de- 
struction. The  tendency  is 
for  a  vessel  to  be  whirled 
around  the  storm  center ; 
and  if  it  happens  to  pass 
through  the  center  or  "eye  of 
the  storm,"  the  sudden  change 
in  the  direction  of  the  wind 
may  come  so  quickly  that  the 
ship  is  not  able  to  adjust  its 
course  in  time  to  prevent  foundering. 

When  these  hurricanes  pass  over  oceanic  islands,  they 
often  cause  much  devastation ;  and  the  destruction  of  sev- 
eral war  vessels  at  the  Samoan  Islands,  in  1889,  was  a  result 
of  one  of  the  South  Pacific  hurricanes.  The  history  of  the 
West  Indies,  and  of  the  southeast  coast  of  Asia,  is  replete 


Fig.  44. 

Diagram  showing  conditions  of  wind 
and  pressure  in  an  actual  hurricane. 


STORMS. 


89 


with  instances  of  the  destruction  of  stout  vessels  that  have 
been  overtaken  by  hurricanes  or  typhoons. 

Accompanying  the  storms,  there  are  often  great  ocean 
waves  which  sweep  over  low-lying  coasts,  sometimes  com- 
pletely destroying  all  life  and  property  in  the  areas  visited. 
On  September  15,  1875,  three-fourths  of  Indianola,  Texas, 


100 


40 


60 50 


""  Halifax  vi^y 


J^-^        Philadelphia 
X>-ashmgtoa^(^/-j         ^ 


/- 


TRACKS  OF  STORMS 

PROCEEDING 

FROM  THE  TROPICS  IN 

AUGUST 

1888 

1889 

1890 

1891h_h 

1892_^4^-^_t.|_H 

1893-)— 


40 


30 


20 


Fig.  45. 
Tracks  of  August  hurricanes,  1888-1893. 


was  destroyed,  176  lives  were  lost,  a  million  dollars'  worth 
of  property  was  destroyed,  and  much  destruction  was  done 
elsewhere  along  the  coast.  The  same  town  was  again  devas- 
tated on  August  19  and  20,  1886.  On  the  Ganges  delta, 
many  hundreds  of  thousands  of  lives  have  been  lost  as  a 
result  of  these  waves.     In  one  storm  alone,  that  of  October 


90  PHYSICAL   GEOGRAPHY, 

31,  1876,  100,000  people  were  killed.  Even  along  the  At- 
lantic coast  of  the  United  States,  where  the  hurricanes  are 
of  much  less  violence  than  in  the  tropics,  a  vast  amount  of 
destruction  is  done  by  them.  Not  only  are  ships  destroyed, 
but  the  low  coasts  are  swept  by  storm  waves  (Fig.  82), 
as  has  frequently  been  the  case  on  the  New  Jersey  coast 
and  on  the  Sea  Islands  of  the  Carolina  coast. 

Path.  —  In  the  North  Atlantic,  the  hurricanes  usually  move 
first  toward  the  northwest,  then  they  curve  and  pass  along 
the  Atlantic  coast  of  the  United  States  until  the  latitude 
of  Cape  Hatteras  is  reached,  when  they  generally  turn  to 
the  right  and  pass  in  a  northeasterly  direction  out  into  the 
Atlantic,  which  they  often  cross  (Fig.  45).  However,  at 
times  they  diverge  from  their  path  and  enter  the  United 
States,  passing  northward  into  Canada.  Thus,  while  we 
usually  experience  only  the  western  part  of  the  hurricane, 
at  times  the  very  center  moves  over  the  Atlantic  coast 
states  (Fig.  42).  In  the  North  Pacific,  their  path  is  about 
the  same ;  but  south  of  the  equator,  instead  of  turning  to 
the  right,  they  are  guided  to  the  left  by  the  deflective 
influence  of  the  earth's  rotation  and  the  prevailing  westerlies. 

The  size  of  these  storms  varies  very  greatly ;  and  while 
sometimes  they  are  very  large,  the  area  covered  by  the 
violent  portion  of  them  is  usually  not  more  than  one  or  two 
hundred  miles  in  diameter.  When  most  violent,  the  area 
of  the  hurricane  is  small,  and  this  is  normally  the  case  near 
the  tropics,  not  far  from  the  place  of  origin.  By  the  time 
they  have  progressed  well  into  the  temperate  latitudes, 
their  area  is  greatly  increased,  and  they  sometimes  cover 
several  hundred  thousand  square  miles.  At  the  same  time 
their  energy  decreases,  and  they  may  even  become  worn  out, 
so  that  they  lose  their  distinctive  features,  particularly  when 
passing  over  the  land. 


STOEMS.  91 

Time  of  Occurrence.  —  Another  notable  feature  connected 
with  hurricanes,  is  the  fact  that  they  occur  most  commonly 
in  certain  months  of  the  year.  Between  the  years  1493  and 
1855,  355  supposed  hurricanes  have  been  recorded  at  the 
West  Indies ;  and  out  of  these,  287  occurred  in  four 
months,  42  in  July,  96  in  August,  80  in  September,  and  69 
in  October.  In  the  regions  south  of  the  equator,  the  hurri- 
canes come  most  commonly  in  the  months  of  the  southern 
autumn  and  late  summer,  or  in  other  words  in  January, 
February,  March,  and  April.  In  the  North  Pacific,  the  time 
of  occurrence  of  the  typhoons  is  the  same  as  that  of  the 
Atlantic  hurricanes.  The  so-called  "  line  storm "  of  the 
Atlantic  coast,  which  is  expected  about  the  middle  of  Sep- 
tember, is  in  reality  one  of  these  hurricanes. 

Cause.  —  In  the  explanation  of  hurricanes  there  are  several 
peculiar  features  which  call  for  consideration.  We  must 
bear  in  mind  that  the  storms  are  whirling  areas  of  air,  in 
which  the  winds  move  violently  in  a  spiral  direction  toward 
a  center,  which  is  a  place  of  ascending  air.  The  whirling  of 
the  winds  is  in  a  uniform  direction  (Figs.  41,  43,  and  44),  in 
the  northern  hemisphere  being  toward  the  left  hand.  The 
storms  begin  over  the  ocean  and  are  by  far  the  most  abun- 
dant in  the  late  summer  or  the  autumn.  Their  path  of 
progression  is  first  toward  the  northwest,  and  then  toward 
the  northeast,  after  having  curved  around  with  a  parabolic 
curve  (Fig.  45).  They  are  found  most  commonly  in  the 
northern  hemisphere  and  appear  to  be  entirely  absent  from 
the  South  Atlantic.  Any  explanation  which  does  not  account 
for  these  peculiarities  cannot  be  satisfactory. 

Since  the  storms  are  confined  to  the  regions  near  the 
tropics,  or  occur  outside  of  them  only  after  having  moved  to 
the  north  or  south,  we  naturally  look  to  the  heat  of  these 
regions  as  the  cause  of  the  storms.     The  warm  air  is  ascend- 


92  PHYSICAL   GEOGRAPHY. 

ing  and  winds  are  blowing  toward  the  place  of  ascent.  As 
a  result  of  the  directly  inflowing  air,  a  whirling  cannot 
be  produced;  and  some  cause  must  be  found  which  will 
originate  the  spiral  motion  of  the  air.  A  possible  cause 
for  this  is  the  deflective  influence  of  the  earth's  rotation ; 
but  ordinarily  this  can  produce  little  effect  near  the 
equator,  because  the  difference  in  the  velocity  of  rotation  of 
different  latitudes  in  this  belt  is  very  slight  (Fig.  21). 
Upon  examining  the  temperature  charts  of  the  world, 
we  find  that  the  heat  equator  is  farthest  from  the  geo- 
graphic equator  in  the  late  summer  and  early  autumn,  and 
that  it  migrates  farthest  from  the  equator  in  the  northern 
hemisphere,  while  in  the  Atlantic  it  is  never  far  south  of  the 
equator. 

When  the  place  of  maximum  heat  is  far  from  the  equator, 
the  influence  of  rotation  will  tend  to  turn  the  winds  to  the 
right  as  they  blow  in  toward  the  place  where  the  air  is 
ascending.  The  farther  these  currents  are  from  the  equator, 
the  more  strongly  is  this  tendency  developed;  and  conse- 
quently those  winds  that  blow  toward  the  equator,  are  turned 
more  than  those  that  move  in  from  the  equator.  Thus  a 
whirl  is  begun,  which  in  the  northern  hemisphere,  always  has 
its  winds  turning  toward  the  left  hand.  This  whirl  may 
best  be  started  in  the  summer  or  late  autumn.  The  con- 
ditions are  never  favorable  to  the  production  of  hurricanes 
in  the  South  Atlantic,  because  the  heat  equator  does  not 
migrate  far  into  that  ocean. 

The  almost  exclusive  development  of  hurricanes  over  the 
oceans,  is  probably  due  to  the  presence  of  moisture-laden 
winds  in  these  regions,  as  well  as  to  the  very  uniform  condi- 
tions that  exist  there.  Water  vapor  is  a  great  storehouse  of 
energy,  and  it  is  estimated  that  the  heat  needed  to  form  a 
pound  of  water  vapor,  would  melt  several  pounds  of  iron. 


STORMS,  93 

When  the  vapor  condenses,  this  heat  adds  to  the  energy  of 
the  storm,  and  thus  violent  storms  form  over  the  ocean,  where 
there  is  much  vapor  in  the  air  ;  but  over  the  land  the  condi- 
tions are  not  so  favorable.  The  condensation  of  the  vapor 
aids  the  air  in  rising,  and  the  very  rising  causes  the  con- 
densation of  more  vapor,  so  that  air  is  drawn  toward  the 
center  with  great  velocity  ;  and  this  is  maintained  for  days, 
and  possibly  for  over  a  week,  by  the  constant  supply  of  the 
necessary  energy  in  the  form  of  heat  which  was  latent,  and 
which  becomes  apparent  when  the  vapor  condenses.  As 
the  storm  progresses  into  colder  latitudes,  its  energy  de- 
creases, and  in  time  it  dies  out. 

We  are  able  to  find  a  satisfactory  explanation  of  the  path 
of  the  hurricane,  in  a  combination  of  the  prevailing  winds 
and  the  earth's  rotation.  The  storm  starts  in  the  trade-wind 
belt,  but  it  rises  above  this  belt  into  the  upper  air  of  the 
anti-trades.  The  one  set  of  winds  tends  to  blow  the  storm 
toward  the  southwest,  the  other  toward  the  northeast  (in  the 
northern  hemisphere),  and  the  hurricane  often  remains  nearly 
stationary  for  a  day  or  two,  as  if  in  doubt  which  way  to 
move.  Eventually  it  begins  to  move  in  a  northwest  direc- 
tion toward  the  land,  and  soon  it  comes  under  the  influence 
of  the  earth's  rotation  and  the  prevailing  westerlies.  This 
increases  in  effect  as  the  path  more  nearly  approaches  a 
northerly  direction,  and  the  storms  generally  turn  in  the 
latitude  of  the  region  between  Florida  and  Cape  Hatteras. 

Temperate  Latitude  Cyclones:  Resemblance  to  Hurricanes.  — 
In  many  respects  these  storms  bear  a  resemblance  to  the 
tropical  cyclones ;  and  until  quite  recently  it  was  common 
among  meteorologists  to  consider  the  two  classes  as  related 
phenomena  dependent  upon  similar  causes.  These  storms 
are  the  ones  which  bring  the  greater  part  of  the  rain  to  the 
northern   United  States,   and  upon  which  depend  most  of 


94 


PHYSICAL   GEOGBAPHT, 


the  weather  changes  of  the  northern  temperate  latitudes. 
The  "  northeast  storms  "  of  New  Enghind,  so  called  because 
they  bear  damp  northeast  winds,  belong  to  this  class.  Every 
part  of  the  east  experiences  them,  and  their  importance  is 
very  great. 

So  close  is  the  resemblance  between  hurricanes  and  tem- 
perate latitude  cyclones,  that  when  the  latter  are  violent,  it 


Fig.  46. 
Map  showing  path  pursued  by  a  storm  and  the  conditions  which  accompany  it. 

is  quite  impossible  to  distinguish  the  two  kinds  of  storms. 
There  is  a  resemblance  in  form,  in  winds,  and  in  general 
behavior  (compare  Figs.  44  and  46).  Both  kinds  of  storms 
are  great  whirling  masses  of  air,  in  which  there  are  clouds 
from  which  rain  falls ;  and  the  storm  area  progresses  from 
one  place  to  another.  The  winds  move  along  a  spiral  track 
toward  a  central  area  of  low  pressure,  where  the  air  is 
apparently  ascending.     In  a  part  of  their  course,  where  they 


STORMS. 


95 


cross  the  North  Atlantic,  the  paths   of   the   two   kinds  of 
storms  are  practically  the  same  (Fig.  48). 

Differences  from  Hurricanes.  —  Notwithstanding  these 
resemblances,  there  are  so  many  differences  that  we  are 
warranted  in  considering  hurricanes  and  temperate  latitude 
cyclones  as  separate  phenomena.  One  of  the  most  striking 
differences  is  that  of  size  ;   for  while  the  hurricanes  usually 


Fig.  47. 

Paths  of  low-pressure  areas,  December,  1892,    Large  figures  show  the  number 
of  the  storms,  the  small  figures  are  days  of  the  month. 

begin  as  small  storms,  they  may  cover  a  large  area  when 
tliey  have  passed  far  into  the  temperate  latitudes ;  but 
the  temperate  latitude  cyclones  may  cover  great  areas  even 
shortly  after  their  formation.  The  cyclonic  disturbances 
may  extend  over  the  entire  eastern  third  of  the  country, 
from  Canada  to  the  Gulf,  and  from  the  Atlantic  to  the 
Mississippi.     The  hurricanes  are  most  violent  shortly  after 


96 


PHYSICAL   GEOGBAPHY. 


tliey  are  formed,  while  the  temperate  latitude  cyclones  often 
develop  violence  as  they  proceed  on  their  course.  While  cy- 
clones may  at  times  become  very  violent,  they  never  attain  the 
intensity  which  is  noticed  in  some  hurricanes.  The  whirling 
of  the  air  in  the  temperate  latitude  cyclones  is  not  so  dis- 
tinct as  in  the  hurricanes  (Figs.  44  and  46),  and,  in  them, 
there  is  rarely  if  ever  a  distinct  "eye." 


Fig.  48. 

Average  storm  tracks.    Relative  abundance  indicated  by  numbers  showing  the 
total  number  between  the  years  1878  and  1887. 

While  hurricanes  are  most  commonly  developed  in  the 
autumn,  temperate  latitude  cyclones  occur  in  all  seasons  of 
the  year,  but  are  most  numerous  and  violent  in  the  winter. 
They  do  not  develop  in  tropical  latitudes,  but  are  formed  in 
various  parts  of  the  temperate  zone.  Some  of  them  begin  in 
the  Pacific  Ocean,  others  start  in  the  southwestern  part  of 
this  country,  while  others  are  first  noticed  in  the  northwest. 


STORMS. 


97 


Their  path  of  progression  does  not  show  the  peculiar 
curving  so  noticeable  in  the  tracks  of  hurricanes ;  but  their 
direction  is  usually  toward  the  east  or  northeast  (Figs.  47 
and  49).  If  they  begin  in  the  Pacific  or  the  northwest,  they 
move  in  an  easterly  direction  across  northern  United  States 
or  southern  Canada ;  and  the  center  very  commonly  passes 
over  the  Great  Lakes  and  down  the  valley  of  the  St.  Law- 


FiG.  49. 

Tracks  of  low-pressure  areas  (both  hurricanes  and  temperate  latitude  cyclones), 
October,  1892.  Number  of  the  storm  indicated  by  large  figures,  dates  by  small 
figures. 

rence.  If  they  have  their  beginning  in  the  southwest  (Fig. 
47),  they  first  move  northward,  then  curving  to  the  right, 
they  pass  out  upon  the  Atlantic. 

The  paths  of  the  hurricanes,  and  nearly  all  of  the  north  tem- 
perate latitude  cyclones,  converge  toward  the  Nova  Scotia- 
Newfoundland  region,  and  then  remain  nearly  parallel  across 
the  Atlantic.     Sometimes  these  storms  begin  in  the  Pacific, 


98  PHYSICAL   GEOGRAPHY. 

and  pass  across  the  United  States,  the  Atlantic,  and  Europe, 
thus  going  nearly  around  the  earth  (Fig.  48).  While  the 
path  of  progression  is  usually  regular,  there  are  many  minor 
irregularities  of  a  peculiar  and  rather  exceptional  nature 
(Fig.  49).     The  origin  of  these  is  not  well  understood. 

Effects.  —  The  effects  of  these  storms  in  northern  United 
States  are  very  important ;  and  they  are  not  confined  to  this 
region,  but  occur  in  Asia,  Europe,  and  the  south  temperate 
latitudes.  In  the  United  States,  the  storms  usually  come 
from  the  west,  and  hence  from  the  interior,  while  in  Europe 
they  come  from  the  ocean.  They  bring  to  us  the  greater 
part  of  our  rain  and  snow ;  they  are  the  main  cause  for 
thunderstorms  and  tornadoes ;  they  produce  many  of  our 
most  striking  winds ;  and  they  are  the  cause  for  many  of 
the  changes  in  temperature  which  we  experience.  The 
warm  south  winds  of  the  winter,  and  the  heated  spells  and 
droughts  of  the  summer,  as  well  as  the  cold  northwest  blasts 
of  winter,  have  their  origin  in  these  cyclonic  disturbances. 
At  times  the  violence  of  the  cyclones  is  so  great  that  much 
destruction  is  accomplished  both  on  the  land  and  on  the 
water.  They  are  particularly  destructive  on  the  ocean,  and 
nearly  every  winter  the  fishing  fleet  and  coasting  vessels 
suffer  from  their  destructiveness. 

Winds.  —  The  winds  of  the  temperate  latitude  cyclones 
vary  in  force,  as  well  as  in  direction.  Some  storms  have 
gentle  winds,  while  in  others  they  are  very  violent ;  and  in  dif- 
ferent parts  of  the  same  storm  the  velocity  may  vary  greatly. 
On  the  land  they  are  usually  less  violent  than  on  the  water, 
because  the  irregularities  tend  to  destroy  them  by  friction. 

If  a  storm  is  passing  over  a  given  place,  the  direction  of 
the  wind  changes  during  its  progress  ;  and  the  points  of  the 
compass  through  which  the  wind  veers,  depend  upon  the 
position  of  the  storm  center.     If  it  is  north  of  the  place  of 


STORMS.  99 

observation,  the  kind  of  change  will  be  very  different  from 
that  which  occurs  when  the  storm  center  is  toward  the 
south.  The  best  way  to  understand  these  changes  is  to 
study  the  weather  maps  and  notice  the  change  of  wind  as 
the  storms  progress  on  their  path. 

Certain  special  kinds  of  winds  are  generated  in  cyclonic 
disturbances.  On  the  southern  side  of  a  storm,  warm  winds 
are  drawn  in  from  southern  latitudes ;  and  in  winter  these 
may  cause  a  snowstorm  to  change  to  rain.  In  Italy,  these 
warm  southern  winds  come  from  the  heated  desert  region 
of  northern  Africa,  and  hence  are  usually  dry.  In  that 
country  they  are  known  as  the  sirocco  ;  and  this  same  type 
of  wind  is  also  developed  in  the  United  States.  Here,  how- 
ever, the  sirocco  is  not  dry,  but  is  generally  warm  and  often 
damp.  In  southern  New  England  it  brings  damp  air  from 
the  Atlantic  Ocean ;  and  this  air  is  warm  because  it  comes 
from  the  area  influenced  by  the  Gulf  Stream. 

A  peculiar  type  of  wind  known  as  the  foehn  is  developed 
in  Switzerland,  where  air  is  drawn  over  the  Alps  by  the 
passage  of  a  storm  center  over  central  Europe.  This  air, 
drawn  over  the  Italian  side  of  the  mountains,  is  caused  to 
give  up  much  of  its  moisture  as  it  rises  and  cools.  It  is 
draAvn  down  the  northern  side  of  the  Alps  with  considerable 
velocity,  and  as  it  descends  it  warms  dynamically.  There- 
fore, the  foehn  is  a  dry  and  very  warm  wind,  which  in  win- 
ter will  often  remove  a  layer  of  snow  by  direct  evapora- 
tion. Its  dryness  is  so  remarkable  that  it  has  been  thought 
to  be  a  hot  breath  from  the  Sahara. 

A  similar  wind  is  caused  by  the  passage  of  storm  centers 
east  of  the  Rocky  Mountains  ;  and  in  that  region  it  is  known 
as  the  chinook.  It  is  developed  along  the  eastern  base  of 
the  Rockies  from  Colorado  to  Montana,  and  its  peculiarities 
are  the  same  as  those  of  the  foehn.     In  the  winter  it  often 


100  PHYSICAL   GEOGRAPHY. 

causes  an  unseasonable  rise  in  the  temperature,  and  snow 
disappears  before  it  with  great  rapidity. 

On  the  western  or  rear  side  of  cyclones,  instead  of  warm 
there  are  cold  winds.  Here  the  air  comes  from  cold  north- 
ern lands,  and  in  a  measure  also  from  the  upper  layers  of  the 
air.  When  very  violent,  these  cold  north  or  northwest  winds 
are  known  as  blizzards^  and  they  often  bear  with  them  vio- 
lent squalls  of  snow.  The  true  home  of  the  blizzard  is  the 
northAvest ;  but  even  in  the  plateau  region  of  central  New 
York,  true  blizzards  of  a  somewhat  milder  form,  often  suc- 
ceed the  severe  winter  snowstorms.  In  Europe,  the  same 
form  of  wind  is  developed ;  and  in  Texas  the  norther  is  a 
wind  of  similar  origin. 

Antic  1/ clones.  —  Between  well-developed  cyclones,  there  are 
usually  areas  of  high  pressure,  which  are  knoAvn  as  anti- 
cyclones. In  these,  the  air  is  slowly  settling  ^  from  upper 
parts  of  the  atmosphere,  and  violent  winds  are  not  produced. 
The  air  is  dry  and  clear,  and  hence  radiation  proceeds  rap- 
idly, so  that  at  night  the  temperatures  often  descend  to  very 
low  degrees. 

While  the  air  in  these  anticyclones  is  quiet,  violent  winds 
are  often  present  at  the  margin,  and  particularly  when  the 
margins  merge  into  the  rear  side  of  cyclones.  Indeed,  there 
seems  to  be  a  certain  association  between  the  cyclones  and 
anticyclones,  as  if  the  down-settling  air  of  the  latter  entered 
as  a  part  of  the  whirl  of  the  former.  These  conditions  give 
us  the  cold  waves  of  winter  and  the  cool  spells  of  summer 
(Figs.  63  and  64). 

Cause.  —  Until  recently  it  was  quite  commonly  believed 
that  the  origin  of  temperate  latitude  cyclones  was  the  same 

1  When  air  settles  slowly,  the  dynamic  heating  is  not  marked.  Hence  this 
settling  air  in  anticyclones  usually  reaches  the  earth  with  a  low  temperature ; 
but  there  has  been  some  warming,  and  the  air  is  not  so  cold  as  when  it  started. 


.»    o 


STOBMS.  10$ 

as  tliat  of  hurricanes.  In  objection  to  this  theory  it  may  be 
said  that  convection  does  not  seem  capable  of  accounting  for 
these  great  disturbances.  In  the  first  place,  they  cover  an 
area  often  having  a  diameter  of  more  than  a  thousand  miles, 
but  extend  to  a  height  of  only  two  or  three  miles.  Moreover, 
they  are  most  violent  and  best  developed  in  winter,  when  con- 
vection is  least  active.  Recent  studies  seem  to  show  that  the 
cause  for  these  storms  is  aloft,  not  at  the  ground. 

While  it  cannot  be  considered  proven  that  convection  is 
not  the  cause,  there  are  so  many  reasons  for  doubting  this 
explanation,  that  it  certainly  cannot  be  accepted ;  and  we 
are  now  without  an  explanation  for  these  remarkable,  though 
common,  atmospheric  disturbances.  They  pass  across  the 
country  like  a  series  of  waves  in  the  air ;  and  it  is  possible 
that  the  great  circumpolar  whirl  is  thus  thrown  into  waves, 
and  that  these  disturbances  are  merely  a  secondary  part  of 
this  planetary  circulation.  Recent  studies  seem  also  to  show 
that  there  is  some  relation  between  them  and  magnetism; 
but  we  cannot  feel  certain  of  these  suggestions. 

The  path  followed  by  these  cyclones  and  anticyclones  is 
easily  explained.  They  are  borne  along  in  the  whirl  of  air 
which  moves  about  the  pole,  and  hence  their  direction  is  from 
west  to  east.  As  a  result  of  the  influence  of  the  earth's  rota- 
tion, upon  the  air  currents  the  storms  are  carried  along  their 
regular  paths.  The  winds  in  the  storms  cause  a  whirling  in 
the  same  direction  as  that  of  the  hurricane,  and  for  the  same 
reason,  —  those  on  the  northern  side  are  most  deflected. 

Secondary  Storms.  —  Aside  from  the  greater  general  dis- 
turbances, there  are  certain  minor  phenomena  of  cyclonic 
storms,  which  attract  much  attention  because  of  their  vio- 
lence. The  two  most  important  of  these  are  thunderstorms 
and  tornadoes. 

Thunderstorms.  —  When   moist   air   rises   as   a   result   of 


102' 


r      C 


PHYSICAL   GEOGRAPHY. 


convection,  if  the  ascent  carries  the  air  high  enough  for  the 
dew-point  to  be  reached,  clouds  may  form  and  rain  fall.  In 
such  cases  electricity  may  be  generated,  and  lightning  and 
thunder  may  accompany  the  rain.  In  the  belt  of  doldrums, 
the  ascent  of  the  moist  air  causes  frequent  thunderstorms 
during  the  day ;  and  in  summer,  the  rising  air  among  the 
mountains  may  cause  the  formation  of  thunderclouds  and 
rains.  In  this  class  of  storm  there  is  no  distinct  whirl,  but  a 
simple  ascent  of  moist  air. 

In  central  and  eastern  United  States,  thunderstorms  are 

common  in  sum- 
mer ;  and  they 
also  are  the  result 
of  uprising  moist 
air.  That  this  is 
so,  is  shown  by 
the  fact  that  they 
occur  almost  ex- 
clusively in  sum- 
mer, and  near  the 
close  of  hot,  sul- 
try days.  On 
these  days,  one 
may  often  witness  the  development  of  such  a  storm,  if  the 
place  of  observation  is  sufficiently  elevated  to  command  a 
wide-extending  view  (Fig.  50).  Clouds  begin  to  develop; 
and  if  they  are  seen  from  below,  their  bases  are  fouud  to  be 
flat,  marking  the  plane  at  which  the  rising  air  reaches  the 
dew-point. 

When  seen  at  one  side,  mound-like  masses  of  clouds,  often 
of  mountainous  heights,  are  found  to  rise  above  the  even 
base.  If  the  observer  is  well  to  one  side  of  the  cloud,  it  will 
be  noticed  that  as  the  storm  develops,  the  form  is  quite  like 


Fig.  50. 
Photograph  of  a  distant  thunderstorm. 


ST0R3IS. 


103 


that  of  an  anvil  (Fig.  50).  At  high  elevations,  the  clouds 
extend  out  in  front  of  the  storm,  marking  the  upper  outflow 
of  the  air.  The  great  elevation  of  the  cloud  mass  is  due  to 
the  fact  that  the  air  continues  to  rise  to  these  heights,  and 
the  vapor  to  condense  as  the  temperature  descends. 

Most  of  our  thunderstorms  are  a  part  of  moderately 
developed  c^^clonic  disturbances,  and  they  occur  most  com- 
monly in  the  southern  part  of  these  storms.  Here  warm 
moist  air  is  being  drawn 
in  toward  the  storm 
center,  and  hence  the 
conditions  favoring  the 
development  of  thun- 
der-storms are  pro- 
duced.     As    the   storm 


center     progresses. 


the 


JULY  29  1886 


Fig.  51. 

Progression  of  a  thunderstorm  in  Massachu- 
setts. The  figures  represent  the  hours  at 
which  tlie  storm  front  reached  the  places 
indicated  by  the  line. 


area  in  which  thunder- 
storms may  develop  al- 
so moves  eastward,  and 
any  single  storm  will  be 
found  to  have  the  same 
path  (Fig.  51).  Some 
thunderstorms  have 
passed  entirely  across 
New  England,  while  others  die  out  after  traveling  a  few 
miles.  Some  pass  over  a  broad  path,  while  the  width  of 
others  is  only  a  few  hundred  yards.  When  the  path  is  long, 
the  storm  may  continue  into  the  night ;  and  most  night  thun- 
derstorms have  originated,  during  the  preceding  afternoon, 
at  some  point  far  to  the  west.  The  rate  of  progression  is 
usually  not  greater  than  40  or  50  miles  an  hour. 

In   the    thunderstorm,  after  the  first  violent  squall,  that 
usually  blows  out  from  the  base  of   the  storm,  the  winds 


104 


PHYSICAL   GEOGRAPHY. 


are  generally  not  violent ;  but  there  is  a  steady  and  often 
heavy  downfall  of  rain,  with  accompanying  thunder  and 
lightning.  In  some  cases  the  downpour  of  rain  is  exces- 
sive ;  and  among  the  mountains  of  the  west,  there  are  often 
such  torrents  of  water  that  the  name  cloudburst  is  given  to 
them.  The  name  is  certainly  warranted,  for  the  water  falls 
in  sheets,  in  a  manner  which  can  be  appreciated  only  after 
having  seen  one.  These  excessive  rains  may  be  due  to  a 
supersaturation  of  the  air. 

Tornadoes  and  Waterspouts.  —  These  extraordinarily  vio- 
lent storms  are  fortunately  small,  local,  and  not  common  in 
most  of  the  country.  Like  the  dust  whirl  of  the  desert,  or 
like  the  hurricane,  they  are  whirling  bodies  of  air,  in  which 
the  winds  blow  toward  a  center,  where  they  rise  (Fig.  52). 
The  winds  blow  at  such  terrific  rates  that  houses  are  torn 

down  and  the  parts  carried 
away  (Fig.  53).  The  news- 
papers furnish  vivid  descrip- 
tions of  them ;  and  while 
they  are  often  exaggerated, 
almost  no  story  concerning 
the  action  of  tornadoes  is 
too  incredible  for  belief.^ 
In  the  center,  where  the  air 
is  ascending,  the  air  pressure 
is  often  so  low  that  a  partial 
vacuum  is  produced ;  and  the  walls  of  houses  may  then  be 
blown  outward  by  the  sudden  expansion  of  the  air  within. 
As  the  tornado  approaches,  it  appears  as  a  great  funnel- 
shaped  column  of  black  cloud  (Fig.  52),  in  which  there  are 
signs  of  violent  commotion.  As  it  comes  nearer,  a  roaring 
noise  is  heard  ;  and  as  the  cloud  overspreads  the  sky,  rain  or 
.  1  In  newspaper  accounts  they  are  usually  called  cyclones. 


Fig.  52. 
View  of  a  tornado. 


ST0B3IS. 


105 


hail  falls ;  but  this  ceases  in  the  violent  part  of  the  tornado, 
where  the  air  is  rising  so  rapidly  that  these  forms  of  water 
cannot  descend.  At  first  there  is  no  wind,  then  suddenly  a 
gale  springs  up,  and  almost  immediately  its  violence  becomes 
so  great  that  houses  and  trees  are  felled.  On  opposite  sides 
of  the  storm  the  wind  moves  spirally  toward  the  center. 

The  tornado  usually  progresses  at  a  rate  of  from  25  to 
40  miles  an  hour.  Its  width  is  rarely  as  great  as  a  mile, 
and  more  often 
only  a  few 
hundred  yards, 
or  even  feet, 
so  that  it  cuts 
a  swathe,  on 
either  side  of 
which  no  de- 
struction is  ac- 
complished. 
The  distance 
traversed  by 
one  of  these 
storms  is  gen- 
erally not  more  than  30  or  40  miles,  and  it  rarely  lasts  more 
than  an  hour.  They  do  not  occur  in  large  numbers  outside 
of  the  central  states  of  the  Mississippi  valley,  although  they 
do  occasionally  occur  in  the  east.  West  of  Dakota  they  are 
not  known.  They  not  uncommonly  occur  in  association 
with  thunderstorms  ;  and  like  these,  they  come  after  hot,  sul- 
try days,  in  areas  covered  by  the  southern  portions  of  cyclonic 
storms.     Their  movement  is  almost  invariably  eastward. 

In  part  at  least,  tornadoes  are  due  to  convection  ;  and  the 
reason  for  their  abundance  in  the  Mississippi  valley  seems 
to  be  that  warm,  moist  air  is  drawn  up  that  valley  toward  the 


1 IG.  53. 
Effect  of  a  tornado  at  Lawrence,  Mass.,  July  26,  1890. 


106 


PHYSICAL   GEOGRAPHY, 


storm  center,  while  above  it  there  is  a  colder  layer  of  eastward 
moving  air.  Therefore  the  conditions  of  the  atmosphere 
are  peculiarly  unstable ;  and  the  increased  heat  caused  by  the 
sun,  starts  an  overturning  which  soon  takes  the  form  of  a 
violent  whirl.     This  is  not  possible  in  the  far  west,  where 

the  lower  air  is  dry; 
and  in  the  east,  the 
atmosphere  is  rarely  in 
a  sufficiently  unstable 
state  for  this  violent 
overturning. 

When  the  tornado 
develops  or  passes  over 
the  sea,  or  over  a  large 
lake,  it  takes  the  form 
of  a  waterspout.  It 
is  doubtful  if  these 
waterspouts  are  col- 
umns of  water,  as  is  often  stated;  but  there  is  probably  a 
conical  wave  in  the  center. 


Fig.  54. 

Distribution  of  tornadoes  1794-1881,  the  in- 
tensity of  shading  showing  greatest  abun- 
dance. Darkest  more  than  35,  medium  shade 
25-35,  lightest  shade  less  than  25. 


REFERENCE   BOOKS. 

Ferrel.  —  A  Popular  Treatise  on  the  Winds.  (For  price,  etc.,  see  refer- 
ence at  end  of  Chapter  IV.) 

Davis. — Whirlwinds,  Cyclones,  and  Tornadoes.  Lee  «&  Shepard,  Boston, 
1884.     24mo.     $0.50.     (Reprinted  from  "  Science.") 

Finley.  — Report  on  the  Characters  of  Six  Hundred  Tornadoes. 
Professional  Papers  No.  7,  U.  S.  Signal  Service,  Washington,  1884. 

Finley.  — Tornadoes.  Hine,  New  York,  1887.  12rao.  $1.00.  (Based 
mainly  upon  previous  publications  in  the  U.  S.  Signal  Service  Reports,  etc.) 

The  Monthly  Weather  Review  and  the  Daily  Weather  Maps,  published 
by  the  Weather  Bureau  at  Washington,  and  tlie  Coast  Pilot,  published 
by  the  Hydrographic  Office  of  the  Navy  Department  at  Washington,  will 
be  found  invaluable  in  laboratory  instruction.  Teachers  who  are  inter- 
ested can  probably  obtain  these  upon  application. 


CHAPTER   VI. 

THE  MOISTURE   OF   THE  ATMOSPHERE. 

Dew.  —  AVlien  the  temperature  of  the  air  descends  far 
enough,  a  point  is  reached  when  there  must  be  a  condensa- 
tion of  some  of  the  contained  moisture,  because  the  ability 
of  the  air  to  carry  water  vapor,  depends  in  large  measure 
upon  the  temperature.  With  dry  air,  the  temperature  must 
be  lowered  much  farther  than  with  damp  or  humid  air  ;  and 
on  the  sultry  days  of  summer,  a  pitcher  of  ice  water  lowers 
the  temperature  of  the  air  in  contact  with  it  sufficiently  to 
cause  the  condensation  of  some  of  the  vapor  on  the  outside 
of  the  pitcher,  which  is  said  to  "sweat." 

When  the  ground  becomes  cold  at  night,  the  lower  air  is 
also  cooled,  and  that  which  is  in  contact  with  the  ground 
may  give  up  some  of  its  vapor  as  dew.  The  temperature  at 
which  this  will  happen,  naturally  depends  upon  the  amount 
of  vapor  in  the  air ;  and  in  the  tropics,  where  the  hot  air  is 
very  humid,  the  amount  of  dew  that  forms  at  night  is  often 
very  great.  Even  the  coolness  of  the  late  afternoon  is  often 
sufficient  to  cause  the  condensation  of  dew  in  the  tropics  ; 
and  during  our  own  summer  days,  one  often  notices  that  the 
grass  is  wet  with  dew  even  before  dark. 

Dew  forms  most  readily  on  those  bodies  that  cool  by  radia- 
tion most  quickly.  Thus  grass  and  leaves  are  dew-covered 
sooner  than  soil.  During  some  nights,  even  when  the  air  is 
quite  humid,  dew  is  not  formed.  By  interfering  with  radia- 
tion, clouds  tend  to  prevent  the  formation  of  dew ;  and  as  a 

107 


108  PHYSICAL   GEOGRAPHY. 

result  of  the  stirring  of  the  air,  and  the  inflow  of  new  sup- 
plies of  air,  wind  tends  to  check  dew  formation.  Because 
the  air  is  more  humid,  dew  is  formed  more  readily  near 
streams  or  swamps  than  in  dry  places.  Dew  is  heavier  in 
valleys  than  on  hills,  partly  because  of  the  greater  damp- 
ness of  the  valleys,  partly  because  cold  air  slides  down  into 
them  from  the  hillsides,  and  partly  because  the  air  in  valleys 
is  more  quiet  than  that  on  the  hilltops. 

While  the  main  cause  for  dew  seems  to  be  condensation 
of  vapor  from  the  air,  recent  studies  show  that  this  is  not 
the  only  cause.  At  all  times  plants  are  furnishing  moisture 
to  the  air  by  transpiration.  Ordinarily  this  is  evaporated ; 
but  at  night  this  evaporation  is  checked,  when  the  air  is 
cooled,  and  its  power  for  evaporation  reduced  because  it  is 
either  saturated  or  has  a  high  relative  humidity.  Then  the 
moisture  forms  drops  of  water  on  the  leaves.  Thus  dew  is 
a  result  of  the  combination  of  two  processes,  in  both  of 
which  the  cooling  of  the  air  by  contact  with  the  earth  is 
the  important  cause. 

Frost.  —  When  the  temperature  of  the  dew-point  is  below 
freezing,  the  condensation  of  vapor  takes  the  form  of  frost. 
It  is  not  frozen  dew,  but  vapor  that  has  become  condensed 
as  a  solid,  instead  of  a  liquid.  In  cause,  and  in  occurrence, 
frost  may  be  described  in  the  same  terms  as  those  used  in 
the  description  of  dew.  However,  the  effect  of  frost  is 
quite  different,  for  it  causes  vegetation  to  suffer,  while  dew 
refreshes  vegetation.  Frost  is  not  likely  to  occur  on  windy 
or  on  cloudy  nights,  and  it  comes  earlier  in  damp  valleys 
than  on  dry  hilltops.  This  is  why  the  autumn  foliage 
first  assumes  its  brilliant  tints  in  the  swamps.  A  cover- 
ing, such  as  a  sheet,  by  interfering  with  radiation,  will 
prevent  a  light  frost ;  and  in  this  way  delicate  plants  may 
be  protected  when  there  is  danger  of  frost. 


THE  MOISTURE  OF  THE  ATMOSPHERE.  109 

Fog.  —  This  is  merely  the  condensation  of  water  vapor 
into  the  form  of  very  tiny  drops,  which  are  so  light  that 
they  do  not  readily  fall  to  the  ground.  When  we  breathe 
the  warm  moist  breath  into  the  cold  air  of  a  Avinter  day,  we 
produce  a  tiny  fog.  Many  of  the  great  ocean  fogs  owe  their 
origin  to  a  similar  cause.  On  the  banks  of  Newfoundland, 
where  the  warm  Gulf  Stream  is  side  by  side  with  the  cold 
Labrador  current,  fogs  are  produced  when  the  winds  of  the 
one  region  pass  over  the  other.  Very  extensive  fogs  are 
thus  caused,  and  this  has  made  that  part  of  the  Atlantic 
famous.  When  warm  air  is  draAvn  northward  toward  storm 
centers,  fogs  are  particularly  liable  to  occur  here.  These 
and  other  ocean  fogs  often  extend  upon  the  land,  as  for  in- 
stance on  the  coasts  of  Maine  and  Nova  Scotia. 

A  fog  sometimes  surrounds  an  iceberg,  because  the  air 
around  it  is  chilled.  Over  the  surface  of  lakes  we  some- 
times see  fogs  developed  by  the  chilling  effects  of  air  cur- 
rents. At  times  the  cool  water  produces  a  fog  by  contact 
with  warm  air.  In  damp  valleys,  a  valley  fog  is  often 
formed  when  the  air  is  chilled  and  the  vapor  condensed 
into  particles.  This  is  particularly  liable  to  happen  during 
nights  when  the  conditions  favoring  a  heavy  dew  are  pres- 
ent. Every  one  must  have  noticed  the  cool  dampness  of 
valleys,  which  is  so  noticeable  in  passing  along  a  hilly  road 
just  after  nightfall.  This  often  increases  until  some  of  the 
dampness  forms  into  fog  particles.  One  often  sees  valley 
fog  among  the  mountains  (Fig.  55),  and  many  clouds  are 
nothing  but  fogs.  As  one  looks  down  upon  a  valley  fog, 
there  is  a  white  rolling  surface,  above  which  may  rise  the 
tree  tops  or  church  steeples,  while  everything  else  is  hidden. 
The  appearance  is  not  unlike  that  which  one  sees  in  the 
mountains  when  above  the  clouds. 

By  furnishing  a  nucleus  about  which  the  vapor  may  con- 


110  PHYSICAL   GEOGBAPHT. 

dense,  "  dust "  particles  are  important  in  the  formation  of 
some  fogs.  It  is  believed  that  the  fogginess  of  London  in 
part  depends  upon  the  large  amount  of  dust  in  the  air. 

Haze.  —  At  times,  and  particularly  during  dry  Aveather, 
a  thin  veil  of  blue  haze  extends  through  the  atmosphere  and 
partly  obscures  the  distant  landscape.  Often  it  is  so  indis- 
tinct that  one  notices  it  only  when  an  unobstructed  view 


Fig.  55. 
Valley  fog  in  the  Himalayas.     Mount  Everest  in  the  background. 

of  far-distant  hills  is  obtained;  but  during  some  days  it 
becomes  so  thick  and  dense,  that  points  near  at  hand  are 
almost  completely  obscured,  and  even  the  sun  loses  its  in- 
tensity, while  the  sky  becomes  dull.  Haze  is  not  damp  like 
fog,  and  there  is  reason  to  question  whether  it  is  often  due 
to  water  particles.  Probably  the  greater  part  of  the  haze 
results  from  dust  in  the  air ;  and  during  droughts  the  air 
is  often  very  hazy,  because  at  such  times  rains  have  not 


THE  MOISTURE  OF  THE  ATMOSPHERE. 


Ill 


occurred  to  clear  the  sky,  and  the  air  is  often  supplied  with 
much  dust  from  forest  fires. 

Mist.  —  At  times  the  air  is  filled  with  minute  particles 
of  water,  which  are  larger  than  those  in  a  fog,  and  which 
therefore  cause  greater  dampness.  The  mist  is  intermediate 
between  fog  and  rain,  and  possibly  it  is  made  of  numerous 
fog  particles  which  have  united. 

Clouds.  —  Clouds  are  composed  of  particles  of  moisture 
due  to  the  condensation  of  water  vapor.  Sometimes  these 
particles  are  very  small,  like  those  in  fog,  at  other  times  they 
are  made  of  mist,  or  even  of  raindrops,  and  in  many  cases 
of  ice  particles  or  snow  crystals.     They  are  formed  when- 


FiG.  56. 
The  banner  cloud,  caused  by  a  moist  wind  blowing  against  a  mountain  peak. 

ever  vapor-laden  air  has  its  temperature  lowered  to  the  dew- 
point  ;   and  this  may  be  caused  in  several  ways. 

When  damp  air  encounters  a  cold  mountain  top,  clouds  are 
formed,  and  these  may  surround  the  mountain  peak  or  ex- 
tend beyond  it  like  a  banner  (Fig.  b6^.  Where  high  mountains 
extend  upward  in  the  path  of  the  trade  winds,  these  banner 
clouds  are  often  produced.  Air  that  is  caused  to  ascend,  fre- 
quently has  its  temperature  lowered  below  the  dew-point ; 
and  when  this  point  is  reached,  clouds  are  formed.  This 
may  happen  when  air  rises  by  convection,  or  when  it  ascends 
land  elevations.  During  the  summer,  and  in  mountains, 
such  clouds  are  commonly  formed.     The  mixture  of  air  of 


112 


PHYSICAL   GEOGRAPHY. 


various  temperatures  often  causes  cloud  formation,  and 
this  appears  to  be  the  origin  of  many  of  the  clouds  of  the 
upper  atmosphere.      In  reality,  fog  is  a  form  of  cloud ;  and 


Fig.  57. 

Photographs  of  five  common  cloud  forms. 

Cirrus. 


Nimhus. 
Cirro-cumulus. 


Cumulus. 
Cumulo-stratus. 


during  storms,  when  the  clouds  are  low,  we  may  find  our- 
selves enveloped  in  a  true  cloud  mist. 

The  forms  of  clouds  are  very  beautiful  and  varied,  and 
the  various  kinds  are  known  under  different  names.      The 


THE  MOISTURE  OF  THE  ATMOSPHERE.  113 

following  is  a  classification  of  clouds  based  partly  upon  their 
form  and  partly  upon  their  elevation  :  — 

Cirrus.  Cumulo-steatus. 

Cirro-stratus.  Nimbus. 

Cirro-cumulus.  Stratus. 
Cumulus. 

The  cirrus  cloud  (Fig.  57)  is  the  highest  form  known,  its 
elevation  often  being  greater  than  five  miles.  It  is  so  high 
that  the  condensation  of  water  vapor  forms  ice  spicules,  and 
this  is  the  reason  why  these  clouds  appear  thin,  white,  hazy, 
and  almost  transparent.  They  drift  along  at  very  rapid  rates, 
and  in  northern  latitudes  usually  move  toward  the  east,  being 
carried  along  in  the  circumpolar  whirl.  Their  form  is 
variable  and  often  remarkable.  They  are  commonly  pro- 
duced by  the  upper  outflow  of  air  in  a  cyclonic  storm. 

At  times  the  cirrus  clouds  occur  in  the  form  of  distinct 
bands,  and  they  are  then  known  under  the  name  of  cirro- 
stratus.  This  form  of  cloud  may  completely  overspread  the 
sky,  but  its  transparency  is  so  great  that  the  sun  is  visible 
through  it,  and  during  such  conditions  of  cloudiness  halos 
and  coronas  are  commonly  formed.  Many  varied  forms  of 
cirrus  clouds  are  recognized,  and  various  names  are  given  to 
them.  Sometimes  they  are  frayed  and  torn  as  if  by  violent 
air  currents.  At  other  times  they  occur  in  bunches,  arranged 
often  in  lines,  as  if  produced  by  waves  of  the  air,  the  groups 
of  clouds  resembling  a  choppy  sea.  When  these  bunches  of 
upper  air  clouds  are  quite  distinct,  they  are  known  as  cir?'o- 
cumulus  (Fig.  57).  Oftentimes  the  sky  is  speckled  with 
these  clouds,  and  then  sailors  call  it  the  mackerel  sky. 

Among  the  most  beautiful  of  clouds  are  those  known  as 
cumulus  (Fig.  57).  They  are  produced  at  a  lower  elevation 
than  the  cirrus,  and  are  often  composed  of  fog  particles  in- 


114  PHYSICAL   GEOGBAPHY. 

stead  of  ice.  When  best  developed,  as  is  the  case  in  summer, 
they  are  the  typical  thunder  heads,  which  rise  from  a  flat 
base,  at  an  elevation  of  about  a  mile,  and  extend  into  the  air, 
often  to  a  height  of  several  thousand  feet  above  this. 
They  consist  of  a  mass  of  rounded,  dome-like  clouds,  which 
often  produce  a  very  fantastic  and  beautiful  effect,  partic- 
ularly when  lighted  by  the  rays  of  the  setting  sun. 

These  clouds  are  common,  every-day  occurrences  in  the 
belt  of  calms,  and  in  summer  they  are  often  produced 
around  mountain  peaks,  and  over  the  heated  lowlands.  In 
these  cases  their  cause  is  the  ascension  of  warm  moist  air ; 
and  during  hot  summer  days  they  may  often  be  seen  to 
form.  Over  the  land,  they  are  much  more  readily  formed 
than  over  the  water,  and  the  presence  of  land  is  often  indi- 
cated by  their  occurrence.  When  sailing  along  the  coast  of 
Florida  in  summer,  the  position  of  the  land  is  often  shown 
by  a  line  of  these  clouds.  At  nighttime,  when  convection 
ceases,  the  clouds  melt  away  and  the  sky  clears. 

Clouds  that  resemble  the  cumulus,  but  differ  from  them  in 
being  more  massive  and  banded,  are  known  under  the  name 
of  cumulo-stratus  (Fig.  57).  When  this  form  of  cloud  is  very 
massive,  so  that  large  parts  of  the  sky  are  covered,  the  name 
stratus  is  applied.  These  often  entirely  overspread  the  sky, 
forming  a  gray,  illy-defined  cloud  mass.  Their  elevation 
is  usually  between  600  and  3000  feet,  but  at  times  they  are 
so  low  that  they  touch  the  earth.  This  is  the  kind  of  cloud 
that  occurs  during  cyclonic  storms,  and  then  they  may  cover 
the  sky  over  an  area  of  thousands  of  square  miles.  When 
rain  is  falling  from  a  cloud,  it  is  known  as  nimhus  (Fig.  57). 

Rain. — There  is  every  gradation  between  dew  and  rain, 
and  raindrops  are  often  made  by  the  union  of  numerous  fog 
particles.  The  exact  means  by  which  these  particles  are 
gathered  together,  cannot  be  stated;  but  perhaps  in  many 


THE  MOISTURE  OF  THE  ATMOSPHERE. 


115 


cases  it  is  the  result  of  the  contact  of  particles  driven  against 
one  another  by  wind,  or  as  a  result  of  their  descent  through 
the  air. 

There  is  a  very  definite  relation  between  clouds  and  rain, 
and  the  causes  which  produce  the  one  form  the  other.  The 
most  important  of  the  causes  are  the  mixture  of  currents 
of  different  temperature,  the  uprising  of  air,  and  the  con- 
tact of  warm  moist  air  with  cold  land  surfaces.  The 
greater  part  of 
the  rain  of  the 
world  falls 
either  (1)  from 
cumulus  clouds, 
or  (2)  from  cy- 
clonic storms, 
or  (3)  where 
moist  winds 
blow  from  the 
water  upon  the 
land.  Away  from  places  where  these  conditions  occur,  the 
rainfall  is  usually  light.  Sometimes,  though  rarely,  rain- 
drops fall  from  a  clear  sky. 

Snow.  —  This  is  the  crystallized  form  assumed  when  water 
vapor  condenses  at  temperatures  below  the  freezing-point ; 
and  the  forms  thus  produced  are  often  very  beautiful  and 
fantastic  (Fig.  58).  There  is  an  intimate  relation  between 
snow  and  rain,  and  the  same  storm  may  produce  snow  on  the 
highlands  and  rain  on  the  lowlands.  Many  of  our  winter 
rainstorms  are  due  to  the  fact  that  the  snow  crystals  have 
been  melted  in  their  downward  passage ;  and  the  damp 
snows  are  a  partial  step  in  this  direction  (Fig.  59).  The 
difference  of  a  few  degrees  thus  produces  a  very  marked 
change.     In  the  one  case  rain  falls  and  speedily  flows  away. 


Fig.  58. 
Photographs  of  actual  snowtlakes. 


116 


PHYSICAL   GEOGRAPHY, 


while  in  the  other  case  a  cold  covering  of  solid  snow  is  laid 
upon  the  land,  perhaps  to  stay  for  months.  The  clouds  of 
the  upper  air  are  mostly  made  of  ice  or  snow,  and  mountain 
peaks  that  extend  into  these  upper  layers,  rarely  receive  any 
other  form  of  precipitation. 

Hail.  —  At    times,   particularly   in   summer,  balls   of   ice 
known  as  hailstones  fall  from  the  clouds,  especially  from 


Fig.  59. 

Photograph  taken  after  a  fall  of  damp  snow,  showing  how  it 

clings  to  vegetation. 


those  accompanying  thunderstorms  and  tornadoes.  They 
are  usually  oval  or  rounded  in  form,  and  are  often  made 
of  successive  shells  of  clear  and  clouded  ice.  The  mode  of 
formation  is  not  known ;  but  there  is  some  reason  for  believ- 
ing that  they  are  formed  in  violently  moving  and  rising  air 
currents,  and  that  this  is  the  reason  why  they  so  commonly 
fall  on  the  margins  of  rather  violent  storms. 


Face  page  117. 


Rai 


DIAGRAMATIC  MAP 

SHOWING 

ALiL  OF  THE  WORL.D 

IN  CENTIMETERS  PER  YEAR 


B.D.stitosi.y.y. 


12. 

e  world. 


THE  MOISTUBE  OF  THE  ATMOSPHERE.  117 

Distribution  of  Rainfall  in  the  World.  —  As  used  here,  the 
term  rainfall  includes  both  rain  and  snow.  In  general  there 
is  a  difference  in  the  amount  of  rainfall  according  to  latitude 
and  altitude.  Since  in  high  latitudes  and  high  altitudes  the 
temperature  of  the  air  is  low,  and  therefore  contains  little 
vapor,  the  amount  of  rain  that  can  be  condensed  in  these 
places  is  less  than  in  the  warm  tropics,  where  the  air  is 
humid.  Still,  there  is  much  variation  in  this  respect,  as  will 
readily  be  seen  by  a  glance  at  Plate  12. 

Without  entering  into  the  subject  in  great  detail,  a  few 
notable  facts  shown  on  this  chart  may  be  pointed  out.  It 
will  be  noticed  that  in  the  belts  where  the  trade  winds  blow 
upon  the  land,  the  rainfall  is  heavy,  while  in  those  places 
where  they  blow  over  the  land,  the  rainfall  is  slight.  Thus, 
as  a  result  of  this,  the  dry  desert  of  the  Sahara  exists  in  the 
same  latitude  with  several  very  rainy  districts. 

Where  the  winds  blow  against  steeply  rising  mountains, 
such  as  the  Himalayas,  the  precipitation  is  very  heavy. 
Even  outside  of  the  trade-wind  belt,  when  the  winds  blow 
from  the  warm  ocean  upon  the  land,  the  amount  of  rainfall 
is  often  very  great.  If  mountains  intercept  these  winds, 
they  are  drained  of  their  moisture,  and  pass  to  the  opposite 
side  as  dry  winds,  producing  deserts.  Thus  there  are  two 
important  causes  for  deserts. 

In  the  belt  of  calms,  where  the  air  is  almost  constantly 
rising  during  the  day,  the  precipitation  is  quite  uniformly 
heavy ;  and  as  these  belts  migrate,  the  rainy  conditions  are 
carried  with  them.  Thus  we  may  have  one  very  wet  season, 
and  an  opposite  dry  season,  when  the  calms  are  replaced  by 
the  trades.  This  is  the  case  on  both  sides  of  the  equator 
in  Africa  and  South  America. 

Usually  the  rainfall  is  heavy  near  the  coast ;  but  where  this 
is  not  the  case,  the  winds  are  blowing  from  the  land  to  the 


118  PHYSICAL   GEOGEAPHY. 

sea.  With  the  seasonal  change  in  the  wind  direction,  some 
coasts  have  a  dry  and  a  wet  season.  In  the  interior  of 
continents,  a  condition  of  relative  dryness  usually  prevails. 
This  is  not  always  a  true  desert  condition,  but  often  one  of 
semi-aridity,  in  which  the  rainfall  is  not  sufficient  for  suc- 
cessful agriculture.  There  may  be  every  gradation  be- 
tween the  humid  country  and  a  desert,  passing  through  the 
stages  of  semi-aridity  and  the  climate  in  which  droughts 
are  common. 

The  greatest  irregularities  of  rainfall  are  noticed  in 
temperate  latitudes ;  and  these  depend  in  part  upon  the 
winds,  the  topography,  the  neighborhood  to  the  sea,  and 
the  occurrence  of  cyclonic  storms.  In  parts  of  the  area, 
nearly  the  entire  rainfall  comes  in  association  with  these 
storms.  Bearing  in  mind  the  previous  discussion  of  tem- 
perature, winds,  and  storms,  the  student  will  be  able  to 
understand  most  of  the  irregularities  in  rainfall  distribution 
indicated  on  the  accompanying  charts. 

Distribution  of  Rainfall  in  the  United  States.  —  In  this 
country  (Plate  13),  most  of  the  features  noticed  on  the  rain- 
fall charts  of  the  world  are  well  illustrated  ;  but  we  have  not 
the  tropical  conditions.  On  the  Texas  coast,  the  inblowing 
trades  of  the  summer  cause  a  heavy  rainfall ;  and  in  Florida, 
much  of  the  rain  depends  upon  the  neighborhood  of  the  warm 
ocean  waters.  The  rainfall  of  the  eastern  coast  is  less  than 
that  of  the  western,  because  in  the  former  the  winds  are 
mostly  from  the  land.  Still,  because  this  region  is  frequently 
visited  by  cyclonic  disturbances,  there  are  no  deserts  pro- 
duced in  the  east. 

From  Florida  to  Maine,  the  rainfall  decreases  quite  uni- 
formly, as  it  should  in  passing  from  warm  to  cooler  regions. 
On  the  western  coast,  the  reverse  is  true,  and  the  most  humid 
part  is  in  the  north,  while  the  southern  portion  is  quite 


CO 

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120 


PHYSICAL    GEOGRAPHY. 


arid.     This  is  due  to  the  fact  that  in  the  northern  part,  the 
winds  blow  from  the  ocean  against  the  mountains. 

Because  of  this,  the  rainfall  also  decreases  very  rapidly 
from  the  immediate  coast  toward  the  interior.  Beyond  the 
mountains  of  the  coast,  the  country  is  either  arid  or  in  a 
truly  desert  condition ;  and  this  extends  even  to  the  plateau 
states  east  of  the  Rocky  Mountains.     Throughout  the  greater 


Fig.  60. 

Rate  of  evaporation  in  the  United  States.    Based  upon  observations  for  a  year, 

in  1887-1888. 

part  of  the  western  half  of  the  country,  the  rainfall  is  very 
slight,  because  there  are  no  great  water  bodies  to  supply  the 
winds  with  moisture.  Even  in  the  states  just  west  of  the 
Mississippi  valley,  the  rainfall  is  light  and  quite  irregular, 
because  the  winds  are  dry.  Here  evaporation  is  rapid,  and 
in  some  parts,  where  the  total  annual  rainfall  is  less  than 
10  inches,  it  amounts  to  100  inches  (Fig.  60). 


THE  MOISTURE  OF  THE  ATMOSPHERE. 


121 


Distribution  of  Snowfall.  —  Over  a  very  large  part  of  the 
earth  s  surface  snow  is  impossible,  and  a  considerable  part  of 
the  human  race  has  never  seen  it.  In  the  United  States, 
snow  falls  nearly  everywhere  except  in  Florida  and  south- 
ern Texas  and  California ;  but  it  is  only  in  high  tem- 
perate and  Arctic  latitudes  that  much  snow  can  fall  upon 


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Fig.  61. 

Monthly  rainfall  in  the  West,  showing  the  heavy  winter  rains  of  Washington, 
in  contrast  with  the  normal  condition  of  heaviest  rainfall  in  summer. 
Also  showing  differences  in  amount  in  inches  of  rain. 

the  lowlands.  Even  under  the  equator  it  may  fall  on  high 
mountain  peaks.  There  is  much  variation  in  the  distribution 
of  snow,  both  from  season  to  season,  and  from  place  to  place. 
Where  the  temperatures  are  low,  the  snow  remains  upon 
the  ground  during  the  winter ;  but  in  many  places  it  stays 
only  for  a  short  time.  In  high  mountains,  where  the  snow- 
fall is  great,  and  where  there  is  very  little  melting,  it  may 


122 


PHYSICAL   GEOGBAPHY. 


:-l-o- 


isSSB 


o 


eg 


produce  glaciers  as  a  result  of  the 
accumulation  of  many  winters'  snow- 
fall. The  same  is  true  in  parts  of 
the  Arctic  and  Antarctic  lands,  and 
in  these  cold  places,  even  the  summer 
precipitation  is  mostly  in  the  form  of 
snow. 

Seasonal  Distribution  of  Rainfall. 
—  Many  parts  of  the  earth  have  dry 
and  wet  seasons ;  and  as  has  already 
been  explained,  this  is  usually  due 
to  a  change  in  wind.  In  equatorial 
Africa,  among  the  headwaters  of  the 
Nile,  the  migration  of  the  belt  of 
calms  causes  such  a  condition,  and 
the  same  is  true  of  the  llanos  of 
Venezuela  and  the  campos  of  Brazil. 
The  blowing  of  the  monsoons  upon 
the  coast  of  Asia,  and  elsewhere, 
causes  very  rainy  conditions  which 
are  quite  absent  when  the  monsood 
winds  blow  from  the  land.  At  Cher- 
rapunji,  where  the  rainfall  is  as  great 
as  500  inches  a  year,  the  amount  fall- 
ing in  December  is  only  0.2  inches, 
while  in  July  it  is  over  130  inches. 
This  excessive  rainfall,  which  is  the 
greatest  on  the  earth,  is  caused  by 
the  blowing  of  the  monsoons  against 
a  steeply  rising  mountain  face.  On  \ 
the  western  coast  of  the  United  \ 
States,  particularly  in  Washington  / 
and  Oregon,  the  winter  rainfall  is-^ 


THE  MOISTURE  OF  THE  ATMOSPHERE.  123 

heavy,  while  in  summer  it  is  light  (Fig.  61).  This  is  due 
to  the  damp  winter  winds  from  the  Pacific. 

In  the  central  and  eastern  states,  the  distribution  of  rain- 
fall is  very  irregular,  and  it  depends  upon  the  nature  and 
frequency  of  cyclonic  storms.  Some  seasons  are  ver}^  dry, 
and  then  droughts  may  occur ;  but  there  is  no  regularity  in 
the  recurrence  of  these  periods.  Fig.  62  illustrates  this 
variation  in  the  western  states. 

Irregularities  of  Rainfall.  —  The  normal  rain  is  a  steady 
and  rather  quiet  downpour ;  but  at  times,  particularly  in 
connection  with  thunderstorms,  the  rainfall  may  be  very 
heavy,  and  then  more  rain  may  fall  in  a  few  minutes  than 
during  an  ordinary  cyclonic  storm  lasting  for  a  day  or  two. 
For  instance,  at  Syracuse,  New  York,  8  inches  of  rain  fell 
in  one  day,  June  8,  1876  ;  and  in  June,  1886,  over  21  inches 
fell  in  24  hours  at  Alexandria,  Louisiana.  The  effect  of 
such  a  sudden  deluge  of  water  in  swelling  the  streams  and 
wearing  away  the  land  is  very  important.  The  cloud-bursts 
of  the  Rocky  Mountains  furnish  other  instances  of  very 
remarkable  rainfalls  occurring  in  a  short  period  of  time. 
Where  the  rains  are  excessive  in  violence,  the  soil  is  some- 
times washed  away  from  steep  slopes,  leaving  the  bare  rock 
exposed  to  the  air.  This  is  the  case  in  that  region  of 
remarkable  rainfall  in  India. 


-*o^ 


REFERENCE   BOOKS. 

TyndalL— The  Forms  of  Water  (International  Scientific  Series).  Appleton 

&  Co.,  New  York,  1872.     12mo.     $1.50. 
Schott.  —  Precipitation,  etc.,  of  the  United  States.     Second  edition, 

1885,  Smithsonian  Contributions  to  Knowledge,  Vol.  XXIV.,  1885. 

Smithsonian  Institution,  "Washington,  D.C.     $6.00. 
Harrington. — Rainfall  and   Snow  of  the  United  States.    Bulletin  0, 

Weather  Bureau,  Washington,  1894.     (Many  valuable  charts.) 


CHAPTER   VII. 

"WEATHER   AND    CLIMATE. 

Weather.  —  Climate  is  the  sum  and  average  of  weather, 
which  includes  the  daily  change  in  temperature,  pressure, 
wind,  rain,  etc.  The  climate  shoAVS  the  general  condition, 
while  weather  deals  with  the  special  instances  of  changes 
in  the  atmosphere.  The  data  obtained  in  a  study  of  the 
weather  furnish  the  basis  for  a  knowledge  of  the  climate, 
and  thus  the  two  subjects  grade  into  one  another.  Already, 
in  the  previous  pages,  much  has  been  said  concerning  weather 
and  climate  ;^  but  now  a  few  statements  upon  the  subject  are 
made  as  a  kind  of  summary. 

Tropical  and  Arctic,  —  There  is  much  difference  in  the 
variety  of  weather  in  various  parts  of  the  earth.  Over  the 
ocean,  the  weather  conditions  are  less  variable  than  on 
the  land,  and  the  greatest  variation  is  found  in  temperate 
latitudes.  Day  after  day,  the  weather  in  the  belt  of  calms  is 
nearly  the  same,  the  clear  nights  being  followed  by  cloudy 
days  with  frequent  rains,  and  the  temperature  being  high 
and  not  very  variable.  In  the  belt  of  trade  winds,  the  air 
moves  rather  steadily  toward  the  equator,  and  the  tempera- 
ture is  high.  When  these  winds  blow  over  the  land,  their 
dryness  produces  desert  conditions ;  and  when  they  blow 
upon   the   land,   heavy   rains   are    caused.     Thunderstorms 

1  Many  of  the  foregoing  figures  and  plates  illustrate  this  chapter  as  well. 

124 


WEATHER  AND   CLIMATE.  125 

may  occur,  and  now  and  then  a  hurricane  may  develop, 
bringing  with  it  violent  winds  and  heavy  rains. 

In  the  polar  regions  the  winter  season  is  marked  by 
uniform  cold,  and  the  storms  always  bring  snow.  During 
the  summer  there  is  no  marked  day  and  night  alternation 
in  temperature  ;  and  although  the  air  is  warmer  than  in 
winter,  the  temperature  is  uniformly  low  and  snowstorms 
may  occur. 

Temperate  Latitude  Weather.  —  Taking  the  United  States 
as  typical  of  the  temperate  latitudes,  we  will  examine  the 
weather  conditions  of  several  sections.  On  the  Pacific  coast, 
north  of  central  California,  the  days  of  summer  are  dry  and 
warm,  and  the  nights  become  quite  cool.  In  the  winter  the 
warm  winds  from  the  Pacific  blow  upon  the  land,  producing 
frequent  rains  during  the  day ;  but  the  temperature  of  the 
day,  and  even  of  the  night,  is  moderate. 

In  the  high  mountains  east  of  this,  the  air  is  cold,  and 
even  the  summer  storms  often  produce  snow  instead  of  rain. 
The  temperature  of  day  and  night  is  low.  In  the  desert 
regions  between  the  mountains,  storms  rarely  occur,  and  the 
air  is  quite  constantly  clear  and  dry.  Occasionally,  espe- 
cially in  summer,  there  are  heavy  thunderstorms,  particu- 
larly among  the  mountains ;  but  in  some  of  the  deserts,  as 
for  instance  that  of  Arizona,  there  is  almost  no  rainfall 
(Plate  13).  During  the  summer  day,  the  ground  and  air 
become  highly  heated,  and  at  night  low  temperatures  are 
produced  by  radiation. 

On  the  plains  of  Dakota,  Montana,  Manitoba,  etc.,  the 
air  is  prevailingly  dry ;  and  during  the  summer,  the  tem- 
perature of  the  day  becomes  high,  while  the  nights  are 
cool.  During  the  winter,  excessively  cold  spells  are  liable 
to  occur,  and  temperatures  as  Ioav  as  —30°  are  not  uncom- 
mon.    This  region  is  subjected  to  the  influence  of  cyclones 


126  PHYSICAL   GEOGBAPHY. 

and  anticyclones,  with  their  accompanying  conditions  of 
rain  or  clear  weather  and  variable  winds.  During  the 
winter,  there  may  be  very  heavy  snowstorms,  and  at  times 
extremely  violent  blizzards ;  and,  following  these,  the  warm 
Chinook  wind  may  cause  an  unseasonable  rise  in  temper- 
ature. Farther  south  similar  conditions  prevail,  but  the 
weather  changes  are  less  intense.  On  the  dry  plains  of 
Texas,  the  temperature  ranges  are  extreme  ;  but  neither  the 
chinook  nor  the  blizzard  occurs,  though  a  cold  norther  some- 
times produces  a  very  severe  weather  change. 

Along  the  coast  of  the  southern  states,  high  temperatures 
are  experienced,  and  the  ranges  from  season  to  season,  and 
from  day  to  night,  are  not  great.  Rainstorms  are  produced 
by  the  blowing  of  the  winds  from  the  warm  ocean  upon  the 
land ;  and  in  the  autumn,  violent  tropical  hurricanes  often 
visit  the  coast.  No  snow  falls,  but  during  the  winter,  when 
a  cold  wave  spreads  over  the  country,  freezing  temperatures 
may  at  times  extend  into  this  belt. 

In  the  more  northern  states  of  the  Mississippi  valley, 
the  weather  of  the  winter  is  cold,  snowstorms  accompany 
cyclonic  disturbances,  and  extremely  low  temperatures  are 
produced  during  anti-cyclonic  conditions.  There  are  great 
daily  temperature  ranges,  as  well  as  some  of  an  irregular  na- 
ture. During  the  summer,  cyclonic  storms  are  less  common, 
and  they  come  at  irregular  intervals,  and  there  may  be  long 
periods  of  drought.  These  droughts  occur  when  the  cyclonic 
disturbances  are  of  moderate  intensity,  and  the  storm  centers 
far  to  the  north,  in  the  Canadian  territory.  During  these 
conditions,  warm  air  is  drawn  in  from  the  south  toward 
the  storm  center,  and  this  raises  the  temperature  but  does 
not  produce  rain.  Under  favorable  conditions,  thunder- 
storms and  tornadoes  may  arise  during  the  passage  of  low- 
pressure  areas. 


WEATHER  AND   CLIMATE. 


127 


In  southern  Canada,  New  York,  and  New  England,  the 
weather  is  very  variable  and  irregular.  In  the  winter,  snow- 
storms occur,  and  these  are  sometimes  very  heavy,  particu- 
larly in  the  northern  part  of  the  area.  Over  a  large  part  of 
the  region,  storms  are  of  sufficient  frequency,  and  the  cold 
sufficiently  intense  and  uniform,  to  allow  the  snows  to  ac- 
cumulate during  the  winter  and  remain  upon  the  ground 


Fig.  63. 

Conditions  of  wind,  pressure  and  temperature  accompanying  a  cold  wave, 

March  14,  1895. 


until  spring.  In  the  southern  part  of  the  district  the  snow- 
storms may  change  to  rain,  or  they  may  be  followed  by  warm 
w^eather,  causing  the  winter  thaws,  as  a  result  of  the 
inblowing  wind  from  the  south,  which  is  drawn  toward  the 
storm  center.  Along  the  coast,  cold  east  winds  are  often 
drawn  from  the  ocean,  particularly  during  storms.  The 
cold  waves  which  often  follow  the  storms,  cover  the  land 
with  a  blanket  of  very  cold  air  (Figs.  63  and  64),  through 


128 


PHYSICAL   GEOGRAPHY. 


which  radiation  proceeds  with  ease,  giving  us  our  coldest 
winter  weather ;  but  the  cold  is  not  so  intense  as  in  the  dry 
interior  area  of  the  northwest. 

In  the  summer,  storms  are  less  frequent  and  less  violent ; 
but  still  they  produce  an  effect  upon  the  weather.  When 
they  are  not  intense,  the  warm  air  drawn  in  from  the  south, 
produces  days  of  excessive  heat  and  sultriness,  during  which 
thunderstorms  may  come ;  or  a  continuation  of  this  condi- 
tion may  cause  summer  droughts.  Along  the  seacoast,  fogs 
are  sometimes  blown  in  upon  the  land,  or  the  cool  sea  breeze 
may  temper  the  heat  of  the  summer  day  (Fig.  39).  Well- 
developed  cyclonic   storms   may   arise:   and  in  the  autumn 


50° 
40° 
30° 
20° 
10° 
0° 

Tues.Mar.l2 

NOON 

March  13 

NOON 

March  14 

NOON 

March  15 

NOON 

March  16 

NOON 

March  17 

NOON 

50'- 
40' 
30° 
20  = 
10' 
0= 

1 

1 

i 

1 

1 

1 

! 

-^       i 

\     i 

j 

L — ^ 

^—^         ^^ 

1 

1 

■^ — ^r 

1 

1 

1 

N ^ 

1 

1 

ISOS 

Fig.  64. 
Sudden  descent  in  temperature  during  passage  of  a  cold  wave  at  Ithaca,  N.  Y. 


these  become  more  frequent,  and  the  region  may  then  be 
visited  by  one  of  the  West  Indian  hurricanes.  During  both 
summer  and  winter,  the  winds  are  verv  variable  in  force  and 
direction. 

This,  which  may  be  considered  the  typical  weather  of  the 
temperate  latitude,  has  for  its  main  feature  extreme  irreg- 
ularity and  variability.  From  place  to  place  there  is  much 
variation ;  and  even  at  the  same  place,  the  weather  of  suc- 
cessive years  is  quite  different.  These  are  essentially  the 
conditions  that  prevail  in  Europe  ;  but  here  the  winds  are 
damper  because  their  prevailing  direction  is  from  the  west, 
and  hence  from  over  the  Atlantic.     Since  there  is  less  land, 


WEATHER  AND   CLIMATE. 


129 


there  is  much  less  variability  in  the  weather  conditions  of 
the  temperate  latitudes  of  the  southern  hemisphere. 

Climate.  —  The  earth  may  be  divided  into  climatic  belts, 
the  three  primarj^  zones  being  based  upon  difference  in  lati- 
tude, and  hence  in  supply  of  solar  energy.     These  zones  are 


■'■.■■:-ys—.-i 


Fig.  65. 

Climatic  zones. 


the  tropical,  temperate,  and  arctic  (Fig.  65^  Speaking 
generally,  the  tropical  climate  is  characterized  by  high  tem- 
peratures throughout  the  year,  the  arctic  by  low  tempera- 
tures in  all  seasons,  and  the  temperate  by  variability,  and  a 
marked  change  in  the  two  opposite  seasons  of  summer  and 
winter.     There  are  many  exceptions  to  this  general  state- 


130  PHYSICAL   GEOGRAPHY. 

ment,  and  each  zone  must  be  subdivided  into  oceanic,  insu- 
lar, interior,  and  upland  climates. 

Tropical  Climate. —  Between  the  tropics,  the  climate  of 
the  oceans  and  coasts  is  mostly  warm  and  equable.  The 
rainfall  is  considerable,  though  to  this  there  are  numerous 
exceptions,  as  for  instance  on  those  coasts  from  which  the 
trade  winds  blow  toward  the  sea.  The  doldrum  belt  is  one 
of  excessive  rain  and  very  uniform  conditions  of  tempera- 
ture ;  but  that  of  the  trade  winds  has  a  more  variable 
climate.  In  the  interior  of  the  continents,  there  is  much 
variation,  though  the  uniform  condition  is  that  of  high  tem- 
perature. The  temperature  ranges  are  greater  than  on  the 
ocean,  and  the  average  temperature  is  also  higher. 

There  is  every  gradation  between  the  regions  of  heavy 
equatorial  rains,  and  deserts.  In  the  belt  of  calms  the  rains 
are  heavy,  while  in  the  trade- wind  belts,  the  dry,  south- 
moving  winds  often  produce  a  truly  desert  condition.  Thus 
the  desert  of  Sahara,  with  a  rainfall  of  less  than  20  inches, 
and  in  some  places  with  almost  no  rainfall,  is  on  the  same 
latitude  with  the  region  of  eastern  Central  America,  where 
the  rainfall  is  over  200  inches.  In  the  narrow  zone  which 
is  alternately  occupied  by  the  belt  of  calms  and  the  trade 
winds,  the  climate  of  one  season  is  dry,  and  that  of  the 
other  is  very  damp. 

As  a  result  of  the  monsoon  condition,  a  peculiar  climate 
is  produced  in  India.  There  are  three  seasons  :  one  cool  and 
dry,  when  the  winds  blow  from  the  interior ;  the  second  hot 
and  dry,  when  the  sun's  heat  becomes  intense  ;  and  the  third 
a  wet  season,  when  the  monsoons  blow  upon  the  land. 

Temperate  Climate.  —  For  the  most  part,  the  climate  of  the 
north  temperate  zone  is  very  variable,  and  the  year  is  divided 
into  two  seasons  of  extremes, — the  summer  and  the  winter, 
—  with  intermediate  seasons  of  spring  and  autumn,  which  are 


WEATHER  AND   CLIMATE.  131 

gradations  between  the  summer  and  winter.  However,  this 
belt  is  divisible  into  several  minor  zones,  of  which  we  may 
consider  five  :  the  west  coast,  the  east  coast,  the  interior,  the 
mountain,  and  the  inter-montane  zones. 

The  climate  of  the  west  coasts  is  comparatively  equable 
and  damp,  because  of  the  influence  of  the  ocean,  from  which 
the  winds  blow  as  a  part  of  the  circumpolar  whirl.  On  the 
eastern  coast,  the  climate  is  largely  influenced  by  the  condi- 
tions of  the  interior,  because  the  wind  comes  from  this  direc- 
tion ;  but  the  neighborhood  of  the  ocean  somewhat  modifies 
the  climate,  and  it  is  not  so  extreme  as  that  of  the  interior. 

As  has  been  said,  the  interior  climate  is  very  extreme ;  and 
the  cyclones  and  anticyclones  which  affect  nearly  the  entire 
temperate  belt,  are  much  more  marked  than  in  other  parts  of 
the  zone.  The  climate  of  the  mountains  resembles  that  of  the 
region  in  which  they  are  situated  ;  but  it  is  colder  and  usually 
more  humid.  Thus  frost-covered  mountains  may  rise  from 
desert  plateaus.  Among  the  high  mountains,  much  of  the 
precipitation  is  in  the  form  of  snow.  Between  the  moun- 
tains, and  on  the  leeward  side  of  them,  arid  and  even  desert 
conditions  result  from  the  fact  that  the  winds  are  drained  of 
their  moisture  in  passing  over  the  mountains. 

This  great  variability  in  condition  is  strikingly  shown  in 
passing  around  the  earth  on  one  of  the  parallels  of  latitude, 
such,  for  instance,  as  that  of  50°  N.,  as  described  by  Davis 
in  his  "Elementary  Meteorology."  Passing  from  the  equable 
climate  of  the  Atlantic,  it  crosses  the  European  continent 
through  regions  so  temperate  that  they  are  densely  popu- 
lated. In  Asia,  great  plateau  deserts  are  encountered,  and 
on  the  Pacific  coast  the  climate  is  quite  severe ;  but  in  that 
ocean  very  equable  conditions  are  found.  The  parallel  enters 
British  Columbia,  where  the  climate  is  moderate  and  moist, 
passes  over  the  high  snow-covered  mountains,  and  crosses 


132  PHYSICAL   GEOGBAPHY. 

the  great  interior  region  of  extreme  cold,  north  of  the  Great 
Lakes,  emerging  across  the  Labrador  peninsula  to  the  Atlan- 
tic, in  the  middle  of  which  the  climate  is  modified  by  the 
warm  Gulf  Stream. 

Arctic  Climate. —  The  arctic  climate  is  one  of  extreme 
and  prolonged  cold,  and  the  ground  is  covered  with  snow 
for  the  greater  part  of  the  year.  During  the  winter,  the  sun 
remains  below  the  horizon,  and  in  summer  it  does  not  set. 
On  high  mountains  which  rise  into  the  cold  layers  of  the 
upper  air,^  many  of  the  conditions  of  the  arctic  climate 
extend  into  the  temperate,  and  even  into  the  tropical 
zones  (Fig.  65} .  Between  the  tropics,  a  temperate  climate  is 
found  at  moderate  elevations  on  the  mountain  sides. 

Minor  Variations. —  Aside  from  the  larger  divisions  of 
climate,  there  are  many  smaller  ones.  The  climate  may 
change  very  perceptibly  even  in  a  short  distance,  as,  for 
instance,  in  going  from  the  southern  sunny  side  of  a 
mountain  to  the  shaded  northern  side  (Fig.  68).  Even 
a  lake  of  moderate  size  may  produce  a  perceptible  influ- 
ence upon  climate ;  and  it  is  not  uncommon  to  find  a  belt 
adapted  to  fruit-raising  whose  boundaries  include  but  a 
small  area. 

Changes  in  Climate. —  There  is  an  abundance  of  geological 
evidence  to  prove  that  there  have  been  great  changes  in 
climate  in  parts  of  the  earth's  surface ;  and  there  is  some 
reason  for  believing  that  there  have  been  changes  in  climate 
within  historical  times.  Recent  studies  in  Europe  seem  to 
show  that  there  is  a  period  of  slight  variation  in  climate, 
extending  over  thirty-six  years.  The  change  is  from  drier 
and  warmer,  to  cooler  and  moister  conditions  ;  and  we  are  at 
present  in  the  midst  of   the  warm,  dry  part  of   the  cycle. 

1  Of  course,  this  applies  only  to  the  cold  ;  for  the  position  of  the  sun  does 
not  resemble  that  of  the  Arctic. 


WEATHER  AND   CLIMATE.  133 

This  is  merely  a  suggestion,  and  cannot  be  accepted  as  a 
definitely  established  fact. 

Of  the  geological  evidence  we  are  more  certain.  During 
the  earlier  ages  of  the  earth's  history,  the  climate  of  the 
globe  seems  to  have  been  more  moderate  and  uniform. 
Fossils  of  animals  and  plants  that  are  at  present  confined  to 
warm  latitudes,  are  found  preserved  in  rocks  which  are  buried 
beneath  perpetual  snow.  When  these  forms  of  life  existed, 
this  part  of  the  earth  must  have  been  much  warmer  than  now. 

On  the  other  hand,  during  one  of  the  recent  periods  of 
geological  history,  arctic  conditions  extended  down  into  tem- 
perate latitudes.  The  north  temperate  zone  has  just  emerged 
from  this  period ;  and  during  its  existence,  sheets  of  ice,  form- 
ing great  continental  glaciers,  extended  down  into  regions 
now  densely  populated.  Northern  Europe  and  northeastern 
United  States  were  covered  by  these  glaciers  (see  Chap- 
ter XVII.).  At  about  the  same  time  that  this  ice  sheet 
extended  over  northeastern  United  States,  the  climate  of 
parts  of  the  Great  Basin  region  of  the  West  was  transformed 
from  an  arid  condition  to  one  of  relative  humidity;  and, 
during  this  time,  great  lakes  existed  where  now  there  are 
.only  desert  plains  and  salt  lakes. 

Perhaps  the  causes  for  these  changes  in  climate  are  to  be 
found  in  conditions  which  we  do  not  as  yet  understand ;  but 
they  may  in  part  be  due  to  variations  in  the  movements  of  the 
earth  about  the  sun.  These  are  too  difficult  for  simple  state- 
ment ;  but  they  depend  upon  slow  changes  in  the  distance 
between  the  sun  and  earth,  and  upon  the  rotation  of  the 
earth's  axis,  known  as  the  precession  of  the  equinoxes.^ 

1  The  teacher  will  find  this  theory  fully  stated  in  Croll's  "Climate  and 
Time,"  to  which  reference  is  made  at  the  end  of  this  chapter.  A  shorter, 
but  very  clear  statement  of  the  theory,  will  be  found  in  Geikie's  "  Text-book 
of  Geology,"  pp.  23-30. 


134  PHYSICAL   GEOGRAPHY. 

Since  climate  varies  so  remarkably  with  differences  in  the 
elevation  of  land,  or  in  the  relation  between  land  and  water, 
it  is  possible  that  changes  of  a  purely  geographic  nature  may 
account  for  some  of  the  variations.  If  large  areas  of  land 
should  be  raised  to  greater  elevations,  or  considerable  tracts 
be  lowered,  or  if  the  ocean  currents  should  have  their  courses 
decidedly  changed,  the  climate  of  parts  of  the  earth  would 
be  very  different  from  the  present.  Such  changes  actually 
have  occurred,  and  in  this  way  some  of  the  climatic  variations 
may  be  explained  ;  but  at  present,  only  hypothesis  can  be 
offered  to  account  for  the  change. 


-K>«- 


REFERENCE   BOOKS. 

Greely.  —  American   Weather.     Dodd,    Mead   &  Co.,   New  York,   1888. 

8vo.  $2.50.  (Valuable  information,  particularly  relating  to  United  States.) 
Abercromby.  —  Weather  (International  Scientific  Series).    Appleton  &  Co., 

New  York,  1887.     12mo.    $1.75.     (Refers  more  particularly  to  Europe.) 
Blanford.  — Climate  and  Weather  of  India.    Macmillan  &  Co.,  New  York, 

1889.     8vo.     §3.50. 
Woeikof.i — Die  Klimate  DER  Erde.    Gostenoble,  Jena,  1887.     8vo.     22  m. 
Hann.  —  Handbuch  der  Klimatologie.     Englehorn,  Stuttgart,  1883.     8vo. 

15  m. 
Croll.  —  Climate  and  Time.     Stanford,  London  (Appleton  &  Co.,  New  York 

agents).     Fourth  edition,  1890.     12mo.     $2.50. 

A  series  of  publications  on  the  climate  of  the  states  and  territories  included 
within  the  arid  belt  of  the  West  contains  much  valuable  information.  It  is  by 
Greely  and  Glassford,  and  was  printed  by  the  Signal  Service  at  Washington. 

There  is  an  admirable  discussion  of  some  of  the  climatic  features  of 
New  York,  by  Turner,  in  the  "Fifth  Annual  Report  of  the  New  York 
Weather  Bureau,"  1894.  This  is  distributed  free  of  cost,  and  application  for 
it  should  be  made  to  the  director.  Professor  E.  A.  Fuertes,  Ithaca,  New  York. 

1  In  most  cases  reference  is  not  made  to  works  published  in  languages  other  than  the  English ; 
but  these  books  are  of  especial  importance.  For  a  much  fuller  bibliography  of  the  literature, 
reference  may  be  made  to  the  author's  larger  book,  now  in  preparation.  The  present  lists  are 
Intended  to  do  no  more  than  to  refer  to  a  few  standard  books  in  which  reliable  information  may 
be  found  upon  the  several  subjects  which  of  necessity  are  very  briefly  treated  in  this  book. 


CHAPTER   VIII. 

GEOGRAPHIC  DISTRIBUTION   OF  ANIMALS  AND  PLANTS, 

General  Statement. — There  are  three  great  zones  occupied 
by  life, — the  air,  the  water,  and  the  land.  None  of  the 
animals  of  the  air  exist  in  that  medium  alone,  but  they  are 
in  part  terrestrial  or  aqueous, —  largely  the  former.  Aerial 
animals  belong  to  several  groups  of  the  animal  kingdom,  but 
for  the  most  part  are  either  birds  or  insects.  On  the  land, 
nearly  all  the  great  groups  are  represented,  though  some  of 
the  truly  aqueous  animals  are  absent.  In  the  ocean,  the 
fishes  and  lower  forms  of  animals  are  predominant,  though 
there  are  groups  of  birds  that  dwell  on  the  ocean  for 
a  greater  part  of  the  time ;  and  some  groups  of  mammals, 
such  as  the  whales  and  seals,  have  adopted  this  zone  for 
their  home,  although  nearly  all  of  their  fellow-mammals 
live  on  the  land.  The  group  of  reptiles  is  also  represented 
in  the  sea,  but  the  land  is  their  main  home.  There  is 
much  difference  between  the  life  in  fresh  and  salt  water 
bodies,  the  main  life  characteristics  being  the  same,  but  with 
many  noticeable  minor  differences.  For  instance,  insects  are 
common  inhabitants  of  the  lakes  and  rivers,  but  are  nearly 
absent  from  the  sea ;  and  the  plant  life  of  lakes  is  much 
more  varied  and  high  in  type  than  that  of  the  ocean,  in 
which  only  a  few  species  allied  to  the  land  plants  are  known 
to  occur. 

The  Ocean. ^  —  The  zones  of  air  and  land  maybe  classed 

1  See  also  Chapter  IX.,  pp.  168-174. 
136 


136  PHYSICAL   GEOGBAPHT. 

together ;  but  the  ocean  is  so  different  that  it  must  be  con- 
sidered separately.  The  mobility  of  the  water,  and  the 
moderate  temperature  ranges  over  the  greater  part  of  the 
ocean,  are  both  favorable  to  the  widespread  distribution  of 
marine  life ;  and  thus  in  great  oceans  we  find  some  species 
ranging  almost  from  one  end  to  the  other.  The  limit  of 
temperature  is  the  main  check  to  the  spread  of  ocean  animals, 
and  this  is  well  illustrated  by  the  distribution  of  reef -build- 
ing corals,  which  are  practically  excluded  from  zones  Avhere 
the  water  temperature  descends  below  68°.  Because  the 
temperature  of  the  ocean  water  descends  as  the  depth  in- 
creases, the  forms  of  life  change  with  the  depth. 

In  the  case  of  ocean  plants,  as  indeed  those  of  fresh-water 
bodies,  depth  is  a  very  important  factor  in  limiting  distri- 
bution. Below  a  depth  of  200  feet  in  the  sea  there  is 
a  practical  absence  of  any  form  of  vegetable  life,  because 
below  this  limit  the  sunlight  is  not  powerful  enough  to 
perform  the  work  which  plants  demand  of  it.  The  plants 
of  the  ocean  are  partly  floating  seaweed,  so  well  illustrated 
in  the  gulf  weed  or  sargassum,  which  drifts  in  the  Gulf 
Stream,  and  accumulates  in  great  areas,  or  Sargasso  Seas, 
in  the  eddies  within  the  whirl  of  the  oceanic  currents. 
Many  of  the  plants  are  attached  along  the  shore  line,  and 
the  most  favorable  place  for  this  is  the  rocky  shore,  which 
furnishes  a  firm  base  for  attachment.  Therefore,  along  rock- 
bound  coasts,  the  area  between  tides  is  covered  with  a  mat 
of  seaweed  (Figs.  89  and  204).  Another  favorable  place  for 
oceanic  vegetation,  is  in  quiet,  partly  enclosed  arms  of  the 
sea,  away  from  the  reach  of  the  waves.  Here  many  forms 
of  plants  exist,  some  of  them  belonging  to  the  true  grasses ; 
and  in  such  places  these  help  to  build  level,  swampy  plains, 
known  as  salt  marshes  (Fig.  206). 

Among   the   most   striking   features  connected   with  the 


GEOGRAPHIC  DISTRIBUTION   OF  ANIMALS,  ETC.     137 

oceanic  life,  are  the  wide  distribution  of  its  species  and  the 
great  abundance  of  individuals.  A  striking  difference  is 
noticed  between  the  animals  existing  in  the  warm  waters  of 
the  tropical  belt,  and  those  occurring  on  the  storm-beaten 
coasts  of  the  cold  temperate  and  arctic  zones.  The  latter 
appear  hardy,  while  the  former  are  often  exquisite  in  their 
beauty  of  color  and  delicacy  of  structure.  There  is  much 
less  difference  in  this  respect  among  the  inhabitants  of  the 
mid-ocean ;  for  here  the  changes  in  physical  conditions  are 
less  marked. 

Fresh  Water.  —  In  fresh- water  bodies  there  is  much  less 
variety  of  life,  and  usually  there  is  not  much  opportunity 
for  the  study  of  distribution.  Land  animals,  notably  insects, 
often  go  to  these  water  bodies  for  purposes  of  breeding ;  and 
in  many  cases  marine  fishes  enter  fresh  water  for  the  same 
purpose.  In  certain .  cases,  owing  to  some  accident,  these 
ocean  animals  find  their  place  of  outlet  cut  off,  and  they 
become  land-locked.  As  a  result  of  this,  we  sometimes  find 
true  ocean  fishes  living  in  lakes.  Sometimes  fresh-water 
lakes  become  transformed  to  salt  lakes,  and  this  change 
gradually  exterminates  the  animals.  Finally,  these  water 
bodies  may  become  dead  seas  in  which  practically  no  life 
exists,  as  is  the  case  in  the  Great  Salt  Lake  and  the  Dead  Sea. 

The  way  in  which  lakes  become  inhabited  is  mainly  by 
the  migration  of  life  along  the  streams,  or  else  by  the 
entrance  of  land  animals ;  and  if  a  pond  is  made  in  the 
course  of  a  stream,  it  will  soon  become  stocked  with  both 
animal  and  plant  life. 

The  Land.  Effect  of  Temperature  and  Moisture. — On  the 
land,  one  of  the  main  factors  determining  distribution  of  life 
is  that  of  temperature.  As  a  result  of  this,  we  find  very 
great  differences  between  the  faunas  and  floras  of  the  tropics, 
and  those  of  arctic  latitudes.     This  difference  affects  both 


138 


PHYSICAL   GEOGEAPHr, 


variety  and  abundance  ;  for  while  there  are  many  vicissitudes 
in  the  colder  zones,  everything  favors  the  development  of  life 
near  the  tropics.  The  animals  of  the  Arctic  must  prepare  them- 
selves for  the  long,  cold  winter,  and  at  all  times  the  condi- 
tions surrounding  them  are  severe.  Only  a  few  forms  of 
mammals  exist  there,  and  these  are  very  hardy  and  well  pro- 


9''«f««J[«?)Ma!i^!?^««««f?»^ 


Fig.  66. 
Near  the  timber  line,  Colorado. 


tected  with  fur.  Many  of  the  mammals,  and  most  of  the  birds, 
leave  the  coldest  part  of  the  zone  during  the  winter;  and  some 
of  the  birds  that  spend  the  summer  within  the  Arctic  circle, 
in  winter  pass  southward  to  the  southern  portion  of  the 
temperate  zone.  Keptiles  are  nearly  absent  from  this  cold 
region,  and  the  few  that  do  exist  there  are  of  small  size.  In 
summer,  the  land  is  clothed  with  vegetation  up  to  the  limits 


GEOGRAPHIC  niSTBIBUTION   OF  ANIMALS,   ETC.     139 

of  perpetual  snow ;  but  there  is  a  very  marked  difference 
between  the  scanty  flora  of  the  Arctic  and  the  luxuriant, 
almost  impassable  tropical  forest.  Within  the  Arctic,  the 
trees  are  prevailingly  evergreens,  and  near  the  snow  fields 
all  trees  disappear,  Avhile  their  place  is  taken  by  shrubs  and 
the  smaller  forms  of  plant  life. 

Intermediate  conditions  exist  within  the  temperate  belt. 


Fig.   67. 
Above  the  snow  line,  Mt.  St.  Elias,  Alaska. 


There  is  a  great  variety  of  plant  and  animal  life,  in  the 
southern  part  merging  into  the  tropical  conditions,  in  the 
northern  portion  assuming  arctic  characteristics.  Many  of 
the  birds  of  the  colder  parts  of  the  zone  migrate  to  the  south 
in  the  winter ;  but  the  insects,  reptiles,  and  many  of  the 
mammals,  spend  a  part  of  the  winter  in  a  torpid  or  dormant 
condition,  in  this  respect  resembling  the  Avinter  behavior 
of  plants.     They  have   adapted   themselves   to   the    annual 


140 


PHYSICAL   GEOGRAPHY, 


climatic    change   from   the    rigorous   winter    to    the   warm 
summer. 

The  influence  of  temperature  upon  the  abundance  and 
kind  of  life,  is  also  illustrated  in  the  ascent  of  mountains 
(Fig.  QQ^.  Within  the  tropics,  one  may  pass  upward  into 
places  where  plants  exist,  which  in  many  respects  resemble 
those  of  the  temperate  zone ;  and  in  this  zone  a  flora  of 
arctic  habit  exists  upon  many  of  the  highest  mountain  tops. 
In  studying  the  distribution  of  animals  and  plants,  altitude 
is  found  to  be  a  very  important  factor ;  and  as  one  ascends 

e  a    mountain    range, 

he  finds  familiar 
plants  and  animals 
disappearing  one  by 
one,  and  their  place 
only  partly  taken  by 
species  which  rapid- 
ly decrease  in  num- 
ber   as    the    ascent 

Effect  of  sunlight  in  raising  the  zone  of  vegetation  COUtmues.  ^  When 
higher  on  the  southwest  than  the  northeast  side  the  SnOW  line  is  ap- 
of  a  mountain.  -,      i    ^t-,.         /^rrN 

proached  (rig.  o7), 
the  limit  of  plant  life  is  practically  found,  although  in  favored 
places  some  few  species  may  extend  even  above  this  line. 
Before  the  snow  line  is  reached,  one  passes  the  timber  line, 
and  goes  from  the  forest-covered  slope  to  one  on  which  trees 
do  not  grow  (Fig.  ^Q).  The  elevation  at  which  this  is 
reached,  varies  with  the  latitude  (see  Fig.  Qt))^  and  even  with 
the  portion  of  the  mountain.  If  one  side  is  exposed  to  cold 
winds,  the  limit  is  reached  at  a  lower  altitude  than  on  the 
opposite  side ;  and  the  same  is  true  of  the  side  of  the  moun- 
tain which  receives  the  least  sunlight,  for  in  such  places  the 
average  temperature  is  lower  than  on  the  sunny  side  (Fig.  68). 


Fig.  68. 


GEOGRAPHIC  DISTRIBUTION   OF  ANIMALS,   ETC.     141 


The  moistness  of  the  climate  also  affects  the  spread  of 
animals  and  plants ;  and  in  absolute  deserts  we  find  ai 
almost  entire  absence  of  life,  while  in  those  arid  regions 
which  are  not  true  deserts,  the  plant  and  animal  life  are 
peculiar,  for  the  conditions  are  very  unfavorable  (Figs.  69 
and  70).  Reptiles  and  a  few  insects,  mammals,  and  birds 
constitute  the  fauna ;  and  the  flora  is  characterized  by 
stunted,  spiny  bushes,  and  a  brown  grass  that  becomes 
transformed  to  hay  as  it  grows  in  the  dry  atmosphere. 
Here  the  cactus  thrives, 
and  other  prickly  and 
unusual  forms  of  vegeta- 
tion exist.  With  abun- 
dant moisture,  vegetable 
and  animal  life  flourish, 
and  this  is  one  of  the 
reasons  for  the  luxuriance 
of  the  tropical  forests; 
and  with  abundant  plant 
growth,  animal  life  also 
becomes  abundant.  How 
marked  this  effect  is,  can  readily  be  understood  by  consider- 
ing the  difference  between  the  sandy  wastes  of  the  Sahara 
(or  the  arid  regions  shown  in  Figs.  69  and  70)  and  the 
tropical  forests  in  the  same  latitudes  (Fig.  71).  While 
these  are  extremes,  even  slight  differences  in  rainfall  will 
cause  marked  changes  in  life. 

Plant  and  Animal  Hahits.  —  Aside  from  those  differences 
among  animals  and  plants  upon  which  the  zoological  and  bo- 
tanical classifications  are  based,  there  are  certain  differences 
in  habit  which  are  of  some  interest  from  the  present  stand- 
point. Plants  are  for  the  most  part  fixed  in  a  definite 
place,  and  the  opportunity  of  distribution  comes  only  from 


142 


PHYSICAL   GEOGRAPHY. 


the  seeds.  Therefore  in  the  study  of  geographic  distribu- 
tion of  plants,  the  seeds  are  of  much  interest.  Some  are 
heavy,  and  these  drop  to  the  ground  close  by  the  tree ;  but 
in  some  cases,  these  heavy  seeds  are  enveloped  in  a  fruit, 
which  is  eaten  together  with  the  seed ;  and  since  the  germ  is 
often  protected  by  an  indigestible  shell,  the  vital  part  of 
these  seeds  may  be  carried  for  long  distances,  and  then  be 
left  upon  the  ground  unharmed.      Some  seeds  cling  to  the 


Fig.   70. 
Arid  land  vegetation  in  the  Canon  of  the  Rio  Grande,  northern  New  Mexico. 


fur  of  animals  and  are  thus  distributed,  and  many  are 
drifted  to  distant  regions  by  the  winds.  In  these,  and  other 
ways,  plants  are  spread  from  one  place  to  another. 

Among  land  animals,  there  are  great  differences  in  habit. 
Some  move  slowly,  others  rapidly,  and  some  are  able  to  fly 
in  the  air.  Most  animals  of  the  land  dwell  on  the  surface ; 
but  for  a  part  of  the  time,  many  make  their  home  either 
in  the  air  or  in  the  water,  and  some  spend  a  part  or  all  of 


GEOGBAPHIC  DISTRIBUTION   OF  ANIMALS,   ETC.     143 

their  time  beneath  the  surface  of  the  ground.  Naturally, 
because  of  these  variations,  there  is  much  difference  in  the 
distribution  of  animals,  and  in  the  means  by  which  they  are 
distributed. 

Life  Zones.  —  As  a  complex  result  of  these  animal  and 
plant  peculiarities,  together  with  their  physical  surround- 
ings, and  certain  inherent  conditions  which  cause  life  to 
grow  and  develop,  there  has  resulted  a  peculiar  distribu- 


FiG.   71. 
The  tropical  forest. 

tion  of  life  on  the  land.  The  most  perfect  adjustment  to 
conditions  is  noticed  upon  the  connected  continents ;  and 
here  we  find  three  great  zones,  the  tropical,  temperate,  and 
arctic,  in  each  of  which  there  are  sub-zones  which  are  due  to 
irregularities  in  climate  or  in  topography  (Fig.  72).  There 
are  mountain  and  desert  irregularities,  as  well  as  others. 

Not  only  do  these  zones  exist  upon  the  several  continents, 
but  quite  different  species,  both  of  animals  and  plants,  char- 
acterize the  separate  continental  areas.     The  plants  and  ani- 


144 


PHYSICAL   GEOGRAPHY, 


mals  of  Europe  are  quite  different  from  those  of  America ; 
of  South  America  from  Africa.^  Yet  there  is  remarkable 
uniformity  in  the  fact  that  the  same  large  groups  are  pres- 
ent in  each,  as  if  by  some  means  there  had  been  an  occa- 
sional connection  or  communication.  Evolution  teaches  us 
that  animals  and  plants  have  been  developing  from  simpler 


Fig.  72. 

Diagrammatic  representation  of  the  life  zones  of  the  United  States,  showing 

influence  of  latitude  and  topography. 

to  higher  forms,  and  we  now  know  considerable  concerning 
the  steps  along  which  this  has  proceeded.  The  fact  of  the 
difference  between  the  life  of  the  several  continents,  shows 
that  there  has  not  been  constant  connection ;  but  the  resem- 
blances prove  that  there  has  been  some  communication. 

1  To  illustrate,  we  have  temperate  and  tropical  zones  in  both  South  America 
and  Africa,  and  in  each  of  these  also  the  subdivisions  of  coast,  desert, 
and  mountain  belts.  But  the  tropical  forest  of  Africa  bears  only  a  general 
resemblance  to  that  of  South  America.  However,  these  resemble  each  other 
much  more  closely  than  do  the  forests  of  tropical  and  arctic  zones. 


GEOGRAPHIC  DISTRIBUTION   OF  ANIMALS,   ETC.     145 

Even  more  strikingly  is  this  proven  by  the  resemblances 
and  differences  between  the  life  of  the  oceanic  islands  and 
that  of  the  continents.  These  land  bodies  are  separated, 
and  in  some  cases  have  always  been  separated,  by  a  great 
ocean  barrier ;  yet  at  times,  as  for  instance  in  the  Bermudas 
and  the  West  Indies,  the  life  of  the  islands  resembles  that 
of  the  neighboring  mainland,  although  numerous  species  are 
absent.  On  the  other  hand,  there  are  many  cases  in  which 
the  insular  life  is  entirely  unlike  that  of  the  mainland.  For 
instance,  the  only  native  mammals  of  New  Zealand,  are  two 
species  of  bat,  which  have  been  able  to  spread  themselves  by 
means  of  flight.  Other  vertebrate  animals  are  scarce  on  this 
land,  and  in  most  cases  the  animals  are  of  peculiar  types. 

The  animals  of  the  East  Indies  are  quite  like  those  ol 
Asia ;  but  Australia,  which  lies  only  a  short  distance  south 
of  these,  forms  a  perfectly  unique  life  zone.  Excepting  a 
few  species  of  bats,  rats,  and  mice,  all  of  the  mammals 
belong  to  peculiar  types,  such  as  the  kangaroo  group,  and 
the  group  to  which  the  duck-bill  belongs,  —  an  animal  which 
combines  the  habits  and  characteristics  of  mammals  with 
that  of  laying  eggs.  The  birds  are  also  peculiar,  including 
among  their  number  many  king-fishers,  parrots,  birds  of 
paradise,  and  other  peculiar  forms. 

The  Spread  of  Life.  —  The  prime  cause  for  the  wide  dis- 
tribution of  land  animals  is  found  in  themselves,  as  a  result 
of  voluntary  movement  from  one  place  to  another.  Many 
birds  may  easily  pass  for  long  distances,  and  in  this  way 
they  may  reach  far-distant  lands ;  but  usually  they  are  couv 
tent  to  stay  in  their  normal  home. 

During  storms  they  may  be  blown  far  from  their  home, 
and  when  they  alight,  it  may  be  upon  some  distant  island,  ot 
in  some  other  place  from  which  return  is  not  easy.  Ships  a 
hundred  miles  from  shore,  often  serve  as  a  resting  place  for 


146  PHYSICAL   GEOGRAPHY, 

some  land  bird  which  is  lost  on  the  open  sea.  In  the  water 
there  are  floating  logs  which  may  serve  as  resting  places, 
and  in  this  way  flying  animals  may  be  distributed.  A  few 
years  ago,  during  a  violent  storm,  a  sea  gull  fell  exhausted 
not  far  from  Ithaca,  New  York,  at  a  distance  of  two  or  three 
hundred  miles  from  its  ocean  home,  which  was  certainly  not 
north  of  Philadelphia.  Naturally,  because  of  this  aid  of  the 
wind,  winged  animals  are  most  widely  distributed. 

Land  animals  that  cannot  fly,  distribute  themselves  by 
moving  over  the  land,  each  generation  pushing  its  frontier 
line  farther  than  the  preceding,  provided  other  conditions 
are  favorable.  As  is  stated  in  the  next  section,  there  are 
certain  limitations  to  this  natural  spread  of  life. 

In  a  second  way,  land  animals  may  be  distributed  by 
accidental  means.  Drifting  in  the  ocean  currents,  there 
are  often  logs  of  wood  which  have  floated  down  some  river 
to  the  sea.  Tree  trunks  from  the  American  coast  are  in  this 
way  borne  to  the  European  coast  and  there  stranded.  Ani- 
mals may  be  carried  upon  these,  and  if  they  survive  the 
journey,  they  may  begin  to  increase  on  some  land  remote 
from  their  old  home.  This  is  particularly  likely  to  happen 
to  animals  like  reptiles,  which  can  live  for  a  long  time  with- 
out water  or  food,  or  to  insects  which  are  in  the  Qg^  or  in 
the  cocoon.  In  rare  cases,  even  the  higher  types  of  life  may 
pass  through  such  a  journey  ;  but  such  animals  must  be 
small,  for  only  these  can  be  thus  floated.  It  is  for  these 
reasons  that  the  large  mammals  are  so  rarely  found  upon 
the  islands  of  the  ocean,  even  though  the  distance  to  the 
mainland  is  slight. 

By  a  change  in  climate,  such  as  that  which  produced  the 
glacial  period,  animals  may  be  forced  to  migrate,  and  in  so 
doing  they  may  cause  other  animals  to  abandon  their  homes. 
When  the  ice  enveloped  the  northeastern  United  States,  the 


GEOGRAPHIC  DISTRIBUTION   OF  ANIMALS,   ETC,     147 

animals  Avere  either  killed  or  driven  away  into  the  southern 
and  more  favorable  regions.  The  effect  of  this  migration 
must  have  been  felt  far  from  the  ice  front ;  and  there  are 
still  signs  of  its  influence  in  the  distribution  of  animals 
and  plants  in  eastern  United  States. 

Barriers  to  the  Spread  of  Life.  — The  great  barrier  is  the 
ocean.  More  effectually  than  any  other  feature  of  the  earth 
it  serves  as  a  check  to  the  spread  of  animals  and  plants.  In 
the  case  of  Australia,  it  has  served  as  an  effectual  check 
upon  the  spread  of  the  large  and  powerful  animals  of  the 
East  Indies,  and  in  a  less  perfect  way,  even  upon  the  more 
easily  distributed  forms  of  life.  Short  arms  of  the  sea  are 
not  so  effectual,  but  even  these  serve  as  a  partial  barrier. 
The  study  of  the  problem  offered  by  the  Australian  fauna, 
leads  to  the  conclusion  that  this  continent  has  not  been  con- 
nected with  the  Asiatic  lands  since  the  higher  animals  began 
to  exist ;  and  in  other  parts  of  the  world,  a  study  of  the  dis- 
tribution of  life,  proves  that  some  of  the  ocean  barriers  of  the 
present  have  not  always  existed. 

Next  to  this  great  oceanic  barrier,  the  most  important 
obstacle  to  the  spread  of  life  is  probably  to  be  found  in  high 
mountain  chains,  such  as  the  Andes  and  the  Rockies.  Many 
animals  and  plants  are  completely  checked  by  these.  Nearly 
the  same  is  true  of  deserts ;  for  if  it  is  not  possible  to  pass 
around  these,  many  species  find  it  impossible  to  pass  from 
one  side  of  them  to  the  other.  In  some  cases  even  large 
rivers  serve  as  a  boundary  line,  separating  a  zone  occupied 
by  a  species  from  one  in  which  it  is  absent. 

Effect  of  Man.  —  The  above  remarks  hold  only  for  the 
natural  distribution.  Now  man  has  come  upon  the  scene  as 
a  disturber  of  the  natural  order,  and  everywhere  in  the 
world  we  see  the  result  of  his  interference.  We  have 
European  and  Asiatic  trees  in  the  garden,  and,  in  some  places, 


148  PHYSICAL   GEOGBAPHY. 

even  in  the  forests.  There  are  foreign  weeds  in  the  field, 
foreign  birds,  insects,  and  mammals  (notably  the  rat),  as 
pests,  or  as  unnoticed  additions  to  the  flora  or  fauna.  The 
ancient  marsupials  are  no  longer  the  most  important  mam- 
malian possessors  of  the  Australian  zone,  but  man  has  caused 
an  invasion  of  their  territory. 

Man  is  killing  here  and  adding  there,  with  the  result  that 
intentionally  or  unintentionally,  he  is  changing  the  life  zones  ; 
but  while  thus  interfering  with  the  natural  spread  of  life, 
and,  in  some  cases,  succeeding  in  domesticating  plants  and 
animals  of  one  zone  to  the  conditions  of  another,  he  is  not 
able  to  disturb  the  great  divisions  of  tropical,  temperate, 
and  arctic,  of  mountain  and  desert.  These  depend  upon 
physical  conditions  of  too  fundamental  importance.  The 
camel  may  be  domesticated  in  the  desert  of  southern  Cali- 
fornia, but  it  cannot  thrive  in  New  England ;  the  tiger 
might  be  introduced  into  South  America,  but  not  into  Scan- 
dinavia ;  the  palm  of  the  central  Pacific  might  be  made  to 
grow  on  the  islands  of  the  central  Atlantic,  but  not  on  the 
slopes  of  the  Rocky  Mountains.  Thus  while  man  will 
greatly  aid  in  the  distribution  of  animals  and  plants,  in 
general  he  will  succeed  only  in  disseminating  them  over 
zones  in  which  the  prevailing  conditions  are  similar. 


-K>«- 


KEFERENCE   BOOKS. 


Wallace. — Island  Life.     Macmillan  &  Co.,   New  York,   second  revised 

edition,  1892.     8vo.     $1.75. 
Wallace. — The  Geographical  Distribution  of  Animals.     (Vols.  I.  and 

II.)     Harper  &  Brothers,  New  York,  1876.     8vo.     $10.00. 


Part  II. 

THE    OCEAN. 


CHAPTER   IX. 

FORM  AND  GENERAL  CHARACTERISTICS  OF  THE  OCEAN. 

Distribution  of  Land  and  Water.  —  A  glance  at  a  globe 
shows  a  very  marked  irregularity  in  the  distribution  of  land 
and  water  in  the  different  hemispheres.  It  is  possible  to 
divide  the  earth  into  two  hemispheres,  in  one  of  which  there 
is  little  land,  while  in  the  other  the  water  area  is  small 
(Fig.  2).  Nearly  three-fourths  of  the  earth's  surface  is 
covered  by  water,  the  total  area  of  water  surface  being  about 
145,000,000  square  miles.  Land  rises  from  the  water  in 
the  form  of  continents  and  islands,  which  differ  greatly  in 
outline  and  topography. 

Composition  of  Ocean  Water.  —  The  ocean  is  between  96 
and  97  per  cent  pure  water,  the  remainder  being  divided 
between  several  salts,  of  which  the  most  abundant  is  com- 
mon salt.  In  addition  to  this  common  salt,  there  is  an 
appreciable  amount  of  chloride  of  magnesium,  carbonate  of 
lime,  some  sulphates,  and  very  minute  quantities  of  other 
substances.  Probably  some  compound  of  every  known 
element  is  dissolved  in  the  ocean,  in  such  minute  quantities 
that  they  can  be  detected  only  by  the  most  careful  analysis. 
In  addition  to  these  slight  impurities,  the  water  has  absorbed 
a  considerable  amount  of  atmospheric  gases.  It  is  upon  this 
that  the  ocean  life  depends. 

In  different  parts  of  the  world,  there  is  a  considerable 
variation  in  the  percentage  of  salt  impurities,  the  range 
being  between  3.3  and  3.73  per  cent.      At  the  same  time 

151 


152  PHYSICAL   GEOGRAPHY. 

Avith  this  change  in  amount  of  salt,  there  is  a  variation  in 
the  density  of  the  water.  Representing  fresh  water  as  1, 
the  average  density  of  sea  water  is  1.026.  There  are  many 
reasons  for  variation  in  the  salinity  of  sea  water.  Where 
rivers  enter  the  ocean,  the  density  is  decreased  by  the  addi- 
tion of  fresh  water ;  and  also  where  rains  are  abundant,  as 
they  are  in  the  belt  of  doldrums,  the  surface  water  has  its 
density  decreased.  On  the  other  hand,  where  evaporation  is 
great,  the  removal  of  the  fresh  water  tends  to  concentrate 
salts  and  therefore  to  increase  the  density.  In  the  Mediter- 
ranean and  the  Red  Sea,  the  ocean  water  is  relatively  dense ; 
and  the  same  is  true  of  the  belts  of  ocean  water  over  which 
the  dry  trade  winds  constantly  blow. 

Color  and  Phosphorescence.  —  The  color  of  the  ocean  is 
naturally  blue.  This  is  partly  due  to  the  fact  that  the  blue- 
ness  of  the  sky  is  reflected  upon  the  water  surface,  and  partly 
to  the  scattering  of  light  rays  which  enter  the  water,  this 
cause  being  analogous  to  that  of  the  blue  color  of  the  sky 
itself.  The  color  of  the  bottom  often  imparts  to  the  water 
a  different  shade  from  the  typical  blue  of  the  ocean ;  and 
where  the  water  is  shallow,  green  shades  are  often  produced. 
The  Red  Sea  owes  its  color  to  the  presence  of  many  minute 
forms  of  vegetation,  belonging  to  the  group  of  Algse,  while 
the  color  of  the  water  near  some  coasts  is  due  to  the  pres- 
ence of  great  quantities  of  mud  brought  down  by  the  river. 

At  times,  particularly  on  quiet  nights,  the  ocean  waters 
are  aglow  with  a  silvery  gleam  of  light,  which  is  known  as 
phosphorescence.  It  is  similar  in  origin  to  the  glow  of  the 
fire-fly  which  we  see  on  warm  summer  nights.  In  the  sur- 
face waters  of  the  ocean,  there  are  countless  millions  of 
microscopic  animals,  nearly  all  of  which  are  able  to  emit 
a  tiny  spark  of  this  strange  light ;  and  their  power  to  do 
this  seems  to  vary  from  time  to  time.     Therefore  on  some 


GENEBAL   CHABACTEBISTICS   OF  THE  OCEAN.       153 

nights  the  surface  is  free  from  this  light,  while  at  other 
times  every  ripple  causes  a  silvery  gleam.  In  rowing  upon 
the  surface  of  the  sea  at  such  times,  a  trail  of  light  follows 
behind  the  boat,  and  drops  of  gleaming  water  fall  from  the 
tips  of  the  oars. 

Exploration  of  the  Ocean  Bottom.  —  It  is  only  recently 
that  the  bottom  of  the  ocean  has  attracted  much  attention. 
Until  thirty  years  ago,  it  was  supposed  that  after  passing 
below  a  depth  of  a  few  hundred  feet,  the  bottom  of  the 
ocean  was  a  great,  uninteresting  desert.  And  thus,  until 
that  time,  we  were  almost  entirely  ignorant  of  the  condi- 
tions existing  on  more  than  one-half  of  the  earth's  surface. 
To  the  naturalists  of  the  time  it  seemed  absolutely  impossi- 
ble that  life  could  exist  in  the  depths  of  the  sea. 

When  oceanic  cables  were  laid,  the  beginning  of  the  study 
of  the  deep  sea  was  made,  and  proof  was  soon  obtained  that 
animals  did  live  in  the  great  ocean  depths.  This  proof  first 
came  from  the  Mediterranean,  where  a  submarine  cable  was 
drawn  to  the  surface  for  repair.  Attached  to  it  were  a 
number  of  animals,  which  therefore  must  have  lived  where 
the  cable  lay ;  and  the  depth  of  water  at  this  place  was 
much  greater  than  the  supposed  limit  of  life.  The  fact 
that  conditions  favoring  the  development  of  animals  proba- 
bly existed  over  the  entire  ocean  bottom,  immediately 
created  a  desire  for  exploration  ;  and  to  this  scientific  inter- 
est was  added  the  practical  one,  which  resulted  from  the 
necessity  of  obtaining  a  knowledge  of  the  physical  features 
of  the  bottom,  in  order  to  make  more  easy  the  extension  of 
oceanic  cables ;  and  soon  governments  began  the  study  of 
the  ocean  bottom. 

Methods  Used  in  Deep-sea  Explorations :  Sounding.  —  In  a 
study  of  the  ocean  bottom,  we  wish  to  discover  something 
concerning  the  life  that  exists  there,  something  about  the 


154 


PHYSICAL   GEOGBAPHY. 


topography,  and  something  concerning  the  kind  of  bottom, 
as  well  as  the  character  of  the  water,  and  the  various  physical 
conditions.  For  this  purpose,  one  thing  is  of  prime  impor- 
tance, namely  the  depth  ;  and  in  every  deep-sea  exploration 
this  is  the  first  fact  obtained. ^  For  this  sounding^  many 
ingenious  contrivances   have   been   invented,  the    one   best 

adapted  to  deep-sea  work  be- 
ing the  Sigsbee  deep-sea  sound- 
ing machine  (Fig.  73).  A 
weight  attached  to  the  end  of  a 
fine  steel  wire,  is  carefully  low- 
ered until  the  bottom  is  reached. 
The  ball  of  the  sounding  ma- 
chine sinks  by  its  own  weight ; 
and  when  it  touches  bottom 
a  jar  is  sent  through  the  wire, 
which  is  felt  even  at  the  sur- 
face. The  entire  machine  is 
very  delicately  constructed,  and 
it  records  ocean  depths  with 
great  accuracy.  The  wire 
used  is  so  fine  that  it  would  be 
impossible  to  draw  the  weight 
back  to  the  surface,  and  the 
instrument  is  so  contrived 
that  this  is  left  behind  w^hen 
it  touches  the  bottom  of  the  ocean  (see  Fig.  73). 

The  weight  is  nothing  but  a  cannon  ball  through  which  a 
hole  had  been  bored.  Into  this  hole  is  placed  a  cylinder, 
which  remains  open  during  the  passage  of  the  weight  to  the 
bottom,  and  which  is  automatically  closed  when  the  line  is 
drawn  in.  Usually  the  bottom  of  the  cylinder  is  covered 
1  Ocean  depths  are  measured  in  fathoms,  a  fathom  being  six  feet. 


Fig.  73. 

Deep-sea  sounding  machine,  with  and 
without  the  sinker. 


GENERAL   CHAEACTERISTICS   OF  THE  OCEAN.        155 


with  wax  or  soap,  to  which  clings  a  sample  of  the  mud  of 
the  ocean  floor ;  so  that  as  the  instrument  is  drawn  to  the 
surface,  we  have  both  water  and  mud  from  the  bottom. 

Near  the  weight  a  thermometer  is  attached  to  the  line  ;  and 
this  is  so  made  that  it  is  inverted 
when  the  wire  is  reeled  in,  and  an 
automatic  register  of  the  tempera- 
ture at  the  time  of  inversion  is  thus 
made.  Very  often  several  ther- 
mometers are  attached  to  the  line 
at  different  distances,  so  that  we 
obtain  a  knowledge  of  the  tempera- 
ture of  the  ocean  water  from  the 
surface  down  to  the  very  base  of 
the  column. 

Dredging.  —  In  order  to  obtain  a 
knowledge  of  the  kind  of  life  that 
exists  in  these  great  ocean  depths, 
another  method,  that  of  dredging, 
must  be  followed.  The  dredge,  or 
deep-sea  trawl  (Fig.  74),  is  an  iron 
frame  several  feet  in  length,  to 
which  is  attached  a  bag  net.  This 
is  lowered  to  the  bottom  and 
dragged  over  it,  usually  for  several 
hours.  The  sounding  apparatus  is 
lowered  perpendicularly ;  but  the 
dredge  is  lowered  to  the  bottom, 
and  then  more  rope  is  reeled  out, 
so  that  it  may  be  kept  upon  the 
bottom  and  dragged  over  it.  This  is  done  partly  by  attach- 
ing weights  to  the  dredge,  and  partly  by  the  natural  sagging 
of  the  wire  rope.     After  the  dredge  has  been  down  for  a 


Fig.  74. 
Deep-sea  trawl. 


156 


PHYSICAL   GEOGRAPHY. 


sufficient  length  of  time,  it  is  drawn  to  the  surface  and  its 
contents  examined. 

Imagine  a  balloon  sailing  through  the  air  at  a  height  of 
three  miles  or  more,  and  dragging  a  frame  a  few  feet  in 
length,  over  a  distance  of  a  few  miles.  If  the  operators  of 
this  apparatus  should  imagine  that,  as  a  result  of  a  few  trials, 
they  had  obtained  a  fair  knowledge  of  the  life  existing  on 
the  surface  of  the  earth,  it  will  readily  be  seen  that  they 
would  be  very  much  mistaken.  All  swiftly  moving  animals 
would  escaj)e,  and  only  those  would  be  taken  which  were 
small  enough  to  enter  the  dredge,  and  so  slow  that  they 
could  not  escape  from  it.  In  a  measure  this  is  true  of  our 
explorations  of  the  deep  sea.  If  large  animals  exist  there, 
our  methods  of  exploration  are  not  calculated  to  discover 

them,  nor  should  we 
expect  to  obtain 
many  animals  that 
are  capable  of  rapid 
movement. 

Topography  of  the 
Ocean  Bottom :  G-en- 
eral.  —  There  is  a 
very  profound  dif- 
ference between  the 

Diagram  contrasting  land  and  ocean  bottom  topog-  outline  01  the  OCCan 

raphy.    a,  a,  a,  land  surface;    B,   B,   height  to  bottom  and  the  fca- 

which  mountain  would  rise  if   denudation   were  ,  f  l       r1 

not  acting ;    c,  c,  undulating  ocean  bottom ;    d,  d,  tures  01  iaUQ,  as  WO 

ocean  sediment   partly  obscuring   mountain  fold  knOW    them    on    the 
E,  E ;  V,  volcanic  cone.  , .  ,         t     i     j  i 

continents.  In  both 
places  the  crust  of  the  earth  is  subjected  to  a  tendency 
to  wrinkle,  and  therefore  to  form  mountain  folds ;  and 
in  both  cases  also,  volcanoes  are  produced.  But  on  the 
land,   there   are   forces    at   work    which    are    absent    from 


Fig.  75. 


GENERAL   CHARACTERISTICS  OF  THE  OCEAN.       157 

the  ocean.  The  rain,  rivers,  changes  in  temperature,  and 
wind,  are  engaged  in  the  combined  action  of  carving  and 
sculpturing  the  land,  the  result  of  which  is  to  make  the 
surface  very  irregular,  and  at  the  same  time  to  gradually- 
lower  it  (a,  a.  Fig.  75).  None  of  these  tendencies  exist  in 
the  ocean. 

The  oceanic  areas  are  the  gathering  grounds  for  the  waste 
of  the  land.  Materials  worn  from  the  continents  are  borne 
to  the  sea  in  rivers,  or  are  wrested  from  the  land  margin  by 
waves,  and  distributed  over  the  sea  bottom.  Materials  car- 
ried in  solution  by  river  waters  also  find  their  way  to  the 
ocean ;  and  from  these  the  animals  that  dwell  in  the  sea,  are 
able  to  take  the  materials  which  they  build  into  their  skel- 
etons, and  which  upon  death  they  leave  as  a  contribution  to 
the  ocean  floor.  Therefore  the  tendency  of  deposition  in 
the  ocean  is  to  smooth  the  surface.  Thus  in  the  sea,  while 
excessive  elevations  are  occasionally  found,  the  general  topog- 
raphy is  remarkably  uniform.  There  are  great  elevations, 
because  nothing  is  present  which  tends  to  destroy  the  diver- 
sities produced ;  but  the  absence  of  the  agents  that  are  carv- 
ing and  sculpturing  the  land,  makes  the  sea  bottom  a  place 
of  great  regularity. 

In  the  ocean,  there  are  prevailing  conditions  of  great,  wide- 
stretching  oceanic  plains  or  plateaus  ;  and  where  there  are 
elevations,  these  are  usually  so  gentle  that  they  would  appear 
to  be  nearly  level.  Occasionally,  where  volcanic  peaks  rise 
in  the  ocean,  we  find  exceptionally  steep  slopes.  The  agents 
of  the  air  are  not  present  to  carry  away  the  materials  which 
are  building  the  cone,  and  therefore  most  of  the  material 
that  is  ejected  is  piled  into  one  mass. 

In  a  distance  of  about  70  miles  from  Porto  Rico,  the  depth 
of  the  ocean  descends  to  4561  fathoms ;  and  in  this  region 
there  is  a  difference  in  elevation  of  fully  30,000  feet  in  a 


158 


PHYSICAL   GEOGRAPHY. 


39°  F. 


distance  of  about  80  miles.     Within  sight  of  the  Bermudas, 

at  a  distance  of  from  10  to  30  miles  from 
land,  the  ocean  reaches  a  depth  of  from 
2700  to  2900  fathoms.  Among  the  oceanic 
islands  of  the  Pacific,  differences  in  eleva- 
tion fully  as  great  as  these  are  frequently 
discovered.  On  the  land  there  are  no 
such  excessive  differences  in  elevation  as 
those  which  exist  among  the  volcanic 
islands  of  the  ocean. 

The  Atlantic  Ocean.  —  Perhaps  the  best 
way  to  obtain  an  idea  of  the  topography 
of  the  Atlantic  Ocean,  is  to  make  a  sec- 
tion across  it,  following  approximately 
the  line  traversed  by  the  oceanic  steamers 
(Fig.   76).     Starting   from   the    shore  of 


100 


2000- 


2500 


2000 


1000 


2500 


2000 


2500 


8000 


3000 


2500 


100 


•  p-H 

-(J 


ft 


GO 

s 

o 


38°  E. 


0!  a> 

0)  O) 


^  U  ^    New  York,  an  even,  gently  sloping  plain 
^  "    is  found  stretching  eastward  to  a  distance 


M 


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t-l 
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ID 

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35°  F. 


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a 

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HI 
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GO 
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o 


of  from  50  to  75  miles.  It  is  almost 
level,  and  its  features  are  quite  like  some 
of  the  very  level  plains  on  the  land.  This 
plain  extends  far  above  the  mouth  of  the 
St.  Lawrence,  including  nearly  all  of  the 
area  between  the  present  New  England 
coast  and  a  line  about  100  miles  from 
the  shore.  South  of  New  York  this  sub- 
marine plain,  or  continental  shelf,  rapidly 
narrows  until  off  the  Carolina  coast  it  is 
a  very  narrow  strip.  Such  a  continental 
shelf  as  this,  is  found  along  the  border  of 
nearly  every  continent  on  the  earth, 
though  in  width  there  is  much  variability 
(Plate  14). 


GENERAL   CHARACTERISTICS  OF  THE  OCEAN.       159 

Passing  eastward,  and  for  a  while  leaving  the  track  of  the 
ocean  steamers  to  the  northward,  a  region  of  very  rapid 
slope  is  encountered.  This  is  known  as  the  continental 
slope,  and  in  many  places  the  rate  of  its  descent  is  as  great 
as  that  of  a  mountain.  In  a  distance  of  a  few  miles,  one 
passes  from  the  edge  of  the  shelf,  whose  depth  is  usually 
about  100  fathoms,  to  oceanic  depths  as  great  as  1000 
fathoms.  After  the  1000-fathom  line  is  reached,  the  exces- 
sive rapidity  of  the  slope  decreases  ;  but  still  the  ocean  depth 
rapidly  increases  to  1500  or  2000  fathoms.  In  a  distance  of 
from  50  to  100  miles,  the  depth  has  increased  from  100  to 
2000  fathoms,  where  the  true  oceanic  plateau  is  reached. 

Almost  the  entire  ocean  is  included  in  this  deep  plateau 
area.  Extending  northward  and  southward,  to  the  Arctic 
and  the  Antarctic  circles,  there  is  a  monotonous,  level  plain, 
with  ocean  depths  varying  between  1000  and  3000  fathoms, 
and  only  rarely  broken  by  some  slight  interruption. 

Passing  eastward,  this  plateau  extends  just  beyond  the 
middle  portion  of  the  ocean,  where  the  bottom  gradually 
begins  to  rise,  forming  the  Mid-Atlantic  Midge.  It  extends 
with  considerable  uniformity  from  Iceland  to  the  southern 
limit  of  the  Atlantic  Ocean ;  but  it  reaches  the  surface  only 
here  and  there,  as  in  Iceland,  the  Azores,  St.  Paul,  Ascen- 
sion, and  Tristan  da  Cunha.  It  is  not  a  continuous  ridge, 
but  an  elevated  portion  of  the  ocean  bottom,  whose  broad 
crest  now  reaches  the  surface,  and  again  is  fully  1000 
fathoms  below  it.  Almost  everywhere  along  this  area,  the 
ocean  depth  is  less  than  in  other  places  far  from  land. 

After  passing  the  crest  of  this  rise  the  depth  again 
increases,  until  soundings  of  over  2000  fathoms  indicate 
another  approach  to  the  great  submarine  plateau.  The 
plateau  on  the  eastern  side  is  less  extensive  than  that  on  the 
western ;  and  as  the  European  coast  is  approached,  the  deep 


160  PHYSICAL   GEOGBAPHT, 

oceanic  plateau  rises  toward  the  continent.  Here  the  con- 
ditions that  were  noticed  off  the  American  shore  are  prac- 
tically repeated.  There  is  a  slope  and  then  a  continental 
shelf,  which  merges  into  the  continent  itself.  In  the  vicinity 
of  the  British  Isles  the  shelf  is  broader  than  it  is  along  the 
coasts  of  France  and  Spain. 

Other  Oceans. — Much  less  is  known  concerning  the  con- 
ditions in  the  depths  of  the  Pacific ;  and  almost  nothing  is 
known  concerning  the  Arctic  and  Antarctic  oceans.  So  far 
as  our  knowledge  of  the  Pacific  and  Indian  oceans  warrants 
any  definite  conclusions,  we  may  say  that  the  conditions  of 
the  Atlantic  are  in  a  general  way  repeated.  The  great 
monotonous  plain  is  more  broken  by  volcanic  peaks ;  and  a 
greater  depth  is  found  in  the  Pacific  than  in  any  other  part 
of  the  ocean  (see  Plate  14).  Depths  greater  than  4000  fath- 
oms have  been  discovered  in  several  places  ;  and  in  one  place, 
near  the  Kurile  Islands,  a  sounding  of  4655  fathoms  was 
made.  The  deepest  place  in  the  Atlantic  (4561  fathoms)  is 
near  Porto  Rico.  It  is  a  noticeable  fact  that  these  excessive 
depths  are  found  close  to  the  land.  While  the  greatest  ele- 
vations occur  on  the  land,  the  average  oceanic  depth  is  very 
much  in  excess  of  the  average  land  elevation  ^ ;  and  the  great 
land  elevations  are  at  a  considerable  distance  from  the  sea, 
so  that  the  elevation  of  the  high  mountain  peak  above  its 
base  is  much  less  than  its  elevation  above  sea  level.  The 
greatest  ocean  depths  descend  almost  directly  from  the  land. 

Topography  near  the  Coast.  —  While  this  description  of 
the  ocean  bottom  will  serve  to  present  the  features  of  the 
deep  sea,  it  does  not  convey  any  idea  concerning  the  irregu- 
larities near  the  coasts.  Along  all  continent  margins,  and 
particularly  among  archipelagoes,  the  form  of  the  bottom  is 

1  The  average  depth  of  the  ocean  is  as  much  as  12,000  feet,  while  the 
average  elevation  of  the  land  above  sea  level  is  not  much  more  than  2000  feet. 


tH 

H 
•< 

Ah 


a 
<o 
o 

o 


Pi 

o 
u 

bo 

a 

o 

09 

a 

o 


162  PHYSICAL    GEOGRAPHY. 

exceedingly  irregular.  Without  entering  into  the  subject 
in  very  great  detail,  these  irregularities  could  not  be  ade- 
quately described ;  and  indeed,  our  knowledge  of  the  larger 
part  of  the  ocean  floor  is  so  slight,  that  as  yet  we  know  only 
the  general  features. 

Temperature  of  the  Ocean  Bottom.  —  In  the  neighborhood 
of  continents,  where  the  depths  of  the  sea  are  relatively 
slight,  the  temperature  is  more  or  less  irregular,  and  deter- 
mined by  local  conditions.  It  changes  with  the  season,  and 
is  influenced  by  the  oceanic  and  tidal  currents,  and  even  by 
the  prevailing  winds. 

After  passing  this  shallow  and  variable  zone,  very  uniform 
temperature  conditions  are  encountered.  As  a  general  state- 
ment, it  may  be  said  that  throughout  the  ocean,  there  is  a 
decrease  in  temperature  with  the  increasing  depth.  Starting 
from  the  variable  zone  of  relatively  high  temperatures,  there 
is  at  first  a  rather  rapid  descent  until  the  zone  of  about  40° 
is  reached  at  a  depth  of  a  few  hundred  fathoms.  After 
this,  there  is  a  very  gradual  descent  in  temperature  (Figs.  76 
and  83),  until  the  cold  becomes  as  great  as  that  of  fresh 
water  at  the  point  of  freezing.  Over  a  very  large  part  of 
the  ocean  bottom  the  temperatures  are  between  32°  and  35°. 
On  some  parts  of  the  ocean  bottom,  particularly  in  the  South 
Atlantic  and  the  South  Pacific,  temperatures  of  32°  and 
even  of  31°  are  found. ^ 

While  this  is  true  as  a  general  statement,  there  are  numer- 
ous exceptions  to  it.  In  the  Mediterranean  (Fig.  77),  there 
is  a  decrease  until  the  level  of  the  bottom  of  the  Straits  of 
Gibraltar  is  reached,  after  which  the  temperature  remains 
uniform,  while  in  the  Atlantic  there  is  a  normal  decrease. 

1  In  this  connection  it  must  be  remembered  that  the  freezing-point  of  salt 
water  is  lower  than  that  of  fresh  water ;  and  therefore  temperatures  lower 
than  that  which  we  call  the  freezing-point  may  be  found  in  the  ocean. 


GENERAL   CHARACTERISTICS   OF  THE  OCEAN.       163 


This  is  because  the  Mediterranean  is  enclosed,  and  its  water 
enters  over  the  Straits,  and  hence  with  the  temperature  at 
that  level.  Also,  in  the  Gulf  of  Mexico,  the  temperature  in 
the  deepest  part  is  only  39|-°,  which  is  the  same  as  that  at 
the  Straits  of  Yucatan.  The  deep  part  of  the  Gulf  of 
Mexico  is  2119  fathoms,  while  that  of  the  Straits  of  Yucatan 
is  only  1127  fathoms.  It  will  be  seen,  therefore,  that  this 
decrease  in  temperature  does  not  depend  upon  increase  in 
depth. 

The  peculiar  distribution  of  temperature  in  the  deep  sea, 
is  probably  due  to  movements  of  water  on  the  bottom.  In 
the  Arctic  regions 
the  cold  water  sinks, 
while  at  the  tropics 
warm  water  rises  ; 
and  there  is  a  con- 
stant passage  of 
water  from  one  of 
these  zones  to  the 
other,  giving  a  sur- 
face movement  to- 
ward the  poles,  and 

a  bottom  movement  toward  the  equator.  If  a  barrier  exists 
in  the  line  of  this  deep-sea  circulation,  the  normal  decrease 
in  temperatures  is  interfered  with.  At  the  Straits  of  Gib- 
raltar, the  water  which  passes  into  the  Mediterranean  does 
not  come  from  the  bottom  of  the  ocean,  but  from  a  level 
determined  by  the  bottom  of  the  Straits. 

Light  on  the  Ocean  Bottom.  —  It  seems  certain  that  sun- 
light cannot  possibly  penetrate  through  several  miles  of  salt 
water ;  and  if  this  is  true,  the  greatest  depths  of  the 
ocean  are  practically  dark,  so  far  as  sunlight  is  concerned. 
Although  it  is  probable  that  no  sunlight  penetrates  to  these 


O  8 


0 

200 

500 

1000 

1500 

2000 

Fig.  77. 

Diagram  showing  the  temperature  peculiarities  of 
the  Mediterranean. 


Atlantic 

68° 

Mediterranean 

75° 

54  » 

55° 

52° 

/<V^%V^>v 

55° 

38° 

/M§SUIK- 

55° 

37°        / 

iilSSiSliit 

\ 

55° 

35^V 

^ 

S,55° 

164  PHYSICAL   GEOGRAPHY, 

zones,  it  still  seems  certain  that  some  kind  of  light  does  exist 
there.  This  conclusion  is  forced  upon  us  by  the  fact  that 
many  of  the  animals  in  the  depths  of  the  sea  have  well- 
developed  eyes ;  and,  further,  that  many  of  them  are  brill- 
iantly colored.  Animals  living  in  dark  caves  become  blind  ; 
and  it  seems  hardly  probable  that  these  inhabitants  of  the 
deep  sea  would  continue  to  develop  eyes  for  ages  after  their 
usefulness  had  ceased. 

Phosphorescence  is  a  possible  source  of  light  on  the  ocean 
floor.  After  nightfall,  whenever  a  dredge-load  of  materials 
is  brought  from  the  deep  sea  to  the  surface,  it  is  aglow  with 
the  dull  white  light  of  phosphorescence.  Each  animal,  each 
particle  of  mud,  gleams  with  this  light. 

Materials  composing  the  Ocean  Floor:  Mechanical  Sedi- 
ments. —  There  are  two  classes  of  substances  spread  over  the 
ocean  bottom  :  one  mainly  derived  from  the  land,  or  from 
fragments  of  rock  emitted  from  volcanoes  ;  the  other,  from 
animals  which  have  lived  in  the  ocean.  The  latter  covers  by 
far  the  greater  part  of  the  ocean  floor.  The  sandy  and 
cla3^ey  fragments  of  rock  which  are  derived  from  the  land, 
are  spread  over  the  bottom  of  the  sea  only  in  the  neighbor- 
hood of  the  coasts. 

Grlohigerina  Ooze.  —  One  of  the  most  striking  facts  con- 
nected with  the  ocean,  is  that  the  floor,  covering  an  area 
greater  than  one-half  that  of  the  entire  earth's  surface,  is 
made  up  of  the  remains  of  minute  animals.  When  seen 
with  the  unaided  eye,  the  deposit  is  a  blue  mud  or  ooze ;  but 
when  examined  with  the  microscope,  it  is  found  to  be  com- 
posed of  fragments  or  entire  shells  of  tiny  animals,  generally 
belonging  to  the  group  of  Foraminifera.  The  most  abun- 
dant of  these  are  members  of  the  genus  Globigerina ;  and 
these  are  so  characteristic  of  the  deposit,  that  it  is  known  as 
the  Globigerina  ooze  (Fig.  78). 


GENERAL   CHARACTEBISTICS   OF  THE  OCEAN.       165 


It  covers  the  greater  portion  of  the  Atlantic,  and  large 
parts  of  the  Pacific  and  Indian  oceans.  Its  rate  of  accumu- 
lation must  be  extremely  sIoav  ; 
for  although  the  animals  which 
compose  it  are  very  abundant 
in  the  surface  waters  of  the 
ocean,  they  are  so  small  that 
it  must  require  long  periods 
of  time  to  form  any  considera- 
ble depth  of  ooze.  Each  par- 
ticle must  depend  upon  the  life 
and  death  of  a  tiny  animal. 
The  chalk  of  England,  and 
other  regions,  is  a  rock  whose 
origin  was  similar  to  that  of  the 
Globigerina  ooze. 

Med  Clay. — At  a  depth  great- 
er than  2000  or  2500  fathoms, 
the  bottom  ooze  changes  its 
character  and  becomes  knoAvn  as 
red  clay.  This  form  of  ocean  deposit  is  particularly  abun- 
dant in  the  Pacific,  although  it  is  not  entirely  absent  from 
the  Atlantic.  It  is  one  of  the  most  remarkable  deposits 
being  made  in  the  ocean.  In  these  great  ocean  depths,  the 
power  of  the  salt  water  to  dissolve  the  lime  of  shells  has 
increased  until  this  substance  is  taken  in  solution  as  the 
shells  drop  from  the  surface.  Therefore  the  insoluble  por- 
tions, of  which  there  are  tiny  amounts  in  every  shell,  are  the 
only  parts  of  the  Globigerina  that  reach  the  bottom.  There- 
fore the  ooze  is  in  part  a  residue  of  the  shell  after  the 
soluble  portions  have  been  removed.  And  if  the  shells  were 
small  at  the  beginning,  how  much  smaller  must  these  tiny 
remnants  be! 


Fig.  78. 

Globigerina  ooze  from  the  ocean 
tottom. 


166  PHYSICAL   GEOGRAPHY. 

It  is  not  exclusively  made  of  the  residue  of  the  shells 
of  surface  animals,  but  contains  contributions  from  other 
sources.  The  most  common  addition  comes  from  pumice 
rocks,  which  were  ejected  from  volcanoes,  and  after  floating 
for  some  time  settled  to  the  ocean  bottom  at  some  distant 
point.  Therefore,  remnants  of  volcanic  ash  or  pumice  are 
common  in  the  red  ooze.  Aside  from  this,  there  are  frag- 
ments of  meteorites  which  have  dropped  to  the  bottom, 
indicating  exceedingly  slow  accumulation.  This  deposit 
covers  an  area  of  over  51,000,000  square  miles,  which  is  a 
little  more  than  that  covered  by  the  Globigerina  ooze.  Each 
kind  of  deposit  covers  an  area  equal  to  about  one-fourth  of 
the  earth's  surface. 

Life  in  the  Ocean :  Pelagic  or  Surface  Faunas.  —  The 
ocean  is  the  great  meeting  ground  of  the  life  of  three 
provinces, — the  air,  the  land,  and  the  water.  Forms  belong- 
ing to  all  the  great  groups  of  the  animal  kingdom  find  it 
possible  to  live  in  the  conditions  which  exist  in  the  ocean. 
There  the  conditions  of  life  are  remarkably  uniform ;  for 
there  are  few  changes  in  temperature,  and  few  variations  such 
as  animals  on  the  land  experience.  Day  after  day,  and  j^ear 
after  year,  the  surrounding  conditions  are  nearly  the  same. 
No  such  difference  exists  between  the  surface  faunas  of  the 
ocean  in  different  latitudes,  as  between  the  land  animals  of 
the  tropics  and  the  temperate  latitudes.  This  is  partly 
because  the  temperature  of  the  water  changes  very  slowly 
and  very  slightly,  and  it  is  also  in  part  due  to  the  fact  that 
the  waters  of  the  ocean  surface  are  in  movement,  so  that  the 
temperatures  of  one  latitude  are  distributed  to  another. 
From  the  tropics,  the  currents  bear  bodies  of  warm  water, 
and  in  them  animals  of  tropical  origin ;  and  these  may  be 
distributed  far  over  the  surface  of  the  ocean. 

So  uniform  are  the  conditions  of  temperature,  that  even 


GENEBAL   CHABACTEBISTICS  OF  THE  OCEAN.       167 

very  slight  differences  will  cause  marked  changes  in  the 
faunas.  In  the  Gulf  Stream,  which  flows  at  a  distance 
of  100  miles  or  more  from  tlie  land,  there  are  found  many 
creatures  of  tropical  origin,  which  cannot  exist  in  the 
colder  waters  near  the  coast.  At  times,  during  strong 
prevailing  winds  from  the  south,  these  creatures  are  driven 
into  the  colder  waters ;  and,  as  a  rule,  they  are  unable  to 
survive  the  change.  The  ocean  surface  is  particularly  favor- 
able to  the  wide  distribution  of  animals.  It  is  constantly  in 
motion,  and  as  a  result  of  this,  hardy  animals  may  be  distrib- 
uted from  one  end  of  an  ocean  to  the  other. 

Many  of  the  oceanic  animals  are  free-swimming  creatures, 
others  are  drifting  animals,  and  still  others  are  attached  to 
floating  objects.  This  last  group  is  particularly  liable  to 
be  found  attached  to  the  floating  seaweed  or  Sargassum, 
which  at  times,  particularly  in  the  eddies  between  the 
ocean  currents,  exists  in  such  abundance  that  these  areas 
are  known  as  sargasso  seas.  All  except  the  largest  of  the 
surface  animals  are  in  a  measure  at  the  mercy  of  the  winds 
or  currents. 

At  the  surface,  and  on  the  ocean  bottom,  there  is  abun- 
dant life.  Between  the  surface  and  the  bottom,  over  the 
greater  part  of  the  ocean,  there  is  a  zone  of  water,  at  least 
two  miles  in  depth,  whose  conditions  as  regards  habitation 
are  not  known.  It  is  the  greatest  unexplored  area  on  the 
earth,  and  we  are  unable  to  say  whether  it  is  a  great  desert 
region,  or  whether  it  is  actually  inhabited.  It  is  exceed- 
ingly difficult  of  exploration ;  but  since  animals  have  been 
found  in  every  explored  nook  of  the  ocean,  and  have  become 
adapted  to  each  place,  it  seems  probable  that  some  have 
found  this  zone  and  have  adapted  themselves  to  it. 

Littoral  or  Shore  Faunas.  —  Along  the  shore  line,  the  con- 
ditions more  closely  resemble  those  of  the  land  than  in  any 


168 


PHYSICAL   GEOGRAPHY, 


other  part  of  the  ocean.  There  is  no  such  monotony  of 
conditions  as  we  find  at  the  surface  of  the  ocean  away  from 
the  land.  But  from  day  to  day,  from  season  to  season,  and 
from  place  to  place,  there  are  very  marked  differences  in  the 
conditions  upon  which  the  animals  depend  for  their  existence 
and  variety.  Here,  as  in  every  part  of  the  ocean,  tempera- 
ture is  a  very  important  cause  for  differences  in  faunas 
and  for  variation  in  animal  forms.  Even  a  few  degrees  of 
temperature   will  cause    a   very  marked  difference   in   the 

-  ™__™_^     .  ™    _™.,         ™™^™™_™ ,-„,™„_,     abundance    and 

variety  of  ani- 
mal life.  This 
is  well  illus- 
trated on  the 
coast  of  Massa- 
chusetts, where 
the  end  of  Cape 
Cod  serves  as 
a  dividing  line 
between  two 
quite  distinct 
faunas,  because 
on  the  northern 
side  of  the  cape 
the  water  is 
cool,  while  on  the  southern  side  it  is  comparatively  warm. 
The  influence  of  the  Gulf  Stream  is  felt  south  of  Cape  Cod, 
while  north  of  it,  in  Massachusetts  Bay,  the  cold  Labrador 
current  reduces  the  temperature. 

Another  limitation  upon  the  spread  of  animals  along  the 
shore,  is  that  of  food  supply.  Perhaps  the  best  illustration 
of  this  is  found  in  coral  regions.  At  the  points  reached  by 
food-bringing  currents,  the  abundance  and  variety  of  life  is 


Fig.  79. 

Coral  reef  on  the  Australian  coast. 


GENERAL   CHABACTERISTICS   OF  THE  OCEAN.       169 

very  great  (Figs.  79  and  207),  and  the  coral  polyps  select 
from  the  water  the  food  that  they  need.  Soon  the  waters 
are  robbed  of  their  food  supply,  and  in  passing  on  are  unable 
to  support  abundant  coral  growth.  It  has  been  noticed 
among  the  coral  reefs,  that  on  one  side  of  a  coral  bar, 
the  polyps  groAv  readily  and  in  great  numbers,  Avhile  on 
the  opposite  side,  they  are  very  scarce  and  not  well  devel- 
oped. In  the  one  case  there  is  an  abundance  of  food,  in  the 
other,  the  food  supply  has  been  taken  from  the  water  by 
those  corals  which  have  the  more  favorable  situation. 

It  follows  from  this  that  circulation  of  water  must  take 
place  in  order  to  bring  fresh  food  supply  to  the  animals 
which  are  fixed  in  one  place,  and  which  are  not  able  to  move 
about  for  the  purpose  of  obtaining  the  food  which  they  need 
for  existence.  Therefore  we  rarely  find  coral  reefs  in  other 
places  than  those  bathed  by  currents. 

The  animals  that  dwell  upon  the  shore  line  are  of  several 
kinds :  those  that  are  free  swimming  and  able  to  move 
about ;  those  that  are  drifted  against  the  shore  by  accident ; 
those  that  crawl  about ;  those  that  are  attached  firmly  to 
the  rocky  coasts  ;  and  those  that  burrow  in  the  clay  and 
sand  which  are  found  in  certain  places.  Since  animals  that 
are  in  the  habit  of  attaching  themselves  permanently  to  one 
place,  can  find  no  opportunity  for  this  attachment  in  places 
where  sand  and  clay  form  the  coast  line,  it  follows  that  as 
a  result  of  the  differences  in  kind  of  rock,  there  may  be  very 
marked  changes  in  the  faunas  from  one  place  to  another. 
On  the  rocky  coast,  the  animals  are  almost  entirely  of  types 
Avhich  are  attached  or  which  crawl  about,  while  on  shores 
that  are  sandy  or  clayey,  the  animals  are  almost  all  of  the 
burrowing  and  crawling  types. 

Faunas  of  the  Ocean  Bottom. — Every  dredge  load  that  is 
brought  to  the  surface  during  deep-sea  exploration,  proves 


170  PHYSICAL   GEOGRAPHY. 

the  presence  of  a  great  pressure  of  water  in  the  depths  of 
the  sea.  The  more  highly  organized  animals,  such  as  the 
true  fishes,  are  unable  to  accommodate  themselves  to  this 
change  in  condition ;  and  when  they  are  drawn  to  the 
surface,  they  are  commonly  broken  by  the  expansion  of 
the  gases  within  the  body.  Their  eyes  protrude  from  the 
head,  the  air  bladder  extends  from  the  mouth,  and  the  skin 
is  cracked  and  fissured.  Thus  while  they  may  live  with 
immense  pressures  upon  every  particle  of  the  body,  they  are 
unable  to  exist  when  the  pressure  is  removed  from  the 
outside,  while  it  still  partly  remains  on  the  inside. 

As  a  result  of  deep-sea  exploration,  it  has  been  found  that 
all  the  ordinary  types  of  marine  animals  exist  on  the  ocean 
bottom,  and  that  in  certain  favorable  places  they  exist  in 
great  variety  and  abundance.  Fishes  of  types  not  unlike 
those  found  at  the  surface,  swim  about  in  the  depths  of  the 
sea  ;  starfishes,  crabs,  and  shrimp,  crawl  over  the  bottom 
ooze ;  shells  not  unlike  those  which  we  find  along  the  sea- 
shore, live  on  the  bottom  or  burrow  into  it ;  and  some  forms 
exist  attached  to  solid  parts  of  the  bottom,  while  others  are 
permanently  attached  by  .means  of  root-like  extensions  of 
the  body,  which  ramify  through  the  mud.  Among  the 
animals  of  the  ocean  bottom,  are  found  certain  types  that 
in  an  earlier  stage  in  the  history  of  the  earth  were  quite 
abundant,  but  which  do  not  now  exist  elsewhere,  —  as  if 
they  had  retreated  to  this  place  as  an  asylum  where  changes 
and  struggles  are  practically  absent. 

As  in  other  portions  of  the  ocean,  temperature  is  the  main 
cause  for  variations  in  the  kind  of  animals  dwelling  on  the 
sea  floor.  A  change  of  one  or  two  degrees  causes  an  almost 
absolute  change  in  the  faunas.  This  is  in  large  part  because 
of  the  unvarying  conditions  of  the  ocean  bottom.  There  is 
no  effect  of  day  and  night,  nor  of  season ;  but  year  after  year, 


GENERAL   CHARACTERISTICS   OF  THE  OCEAN.       171 

and  age  after  age,  the  conditions  of  temperature  remain  the 
same.  Therefore  animals  which  have  become  accustomed 
to  a  practically  permanent  condition  of  35°,  will  find  a  de- 
crease in  temperature  to  33°  so  great  that  they  cannot 
survive  the  change. 

Since  these  deep-sea  animals  live  amid  conditions  of  un- 
varying temperature,  there  is  naturally  a  very  great  decrease 
in  vitality  as  the  temperature  decreases.  And  with  perma- 
nent temperature  conditions  of  32°  (or  as  in  some  cases  even 
of  31°),  the  possibility  for  the  existence  of  life  becomes  very 
much  decreased.  Therefore  in  the  coldest  zones  of  the 
ocean,  the  abundance  of  animals  is  not  great. 

Another  feature  upon  which  the  life  of  the  ocean  bottom 
depends,  is  that  of  food  supply.  So  far  as  we  are  able  to 
judge,  the  animals  of  the  ocean  bottom  exist  partly  upon 
one  another,  but  mainly  and  ultimately  upon  a  supply  of 
food  that  rains  down  upon  them  from  above.  The  death 
of  the  animals  of  the  surface  constantly  supplies  the  bot- 
tom creatures  with  the  necessary  food.  As  it  sinks,  each 
tiny  Globigerina  serves  as  a  morsel  for  some  animal  of  the 
ocean  bottom ;  and  the  lack  of  abundance  of  this  kind  of 
food  supply,  seems  to  place  a  limitation  upon  the  excessive 
development  of  animals  on  the  ocean  floor.  This  is  probably 
one  of  the  reasons  why  the  variety  and  abundance  of  the 
bottom  animals  is  not  greater.  There  is  not  food  enough 
for  many  more  to  exist. 

The  animals  of  the  ocean  depend  upon  a  supply  of  oxygen 
for  breathing ;  and  this  is  as  true  of  the  animals  of  the  ocean 
bottom  as  it  is  of  those  at  the  surface.  It  is  not  difficult  to 
understand  how  the  creatures  that  dwell  in  the  surface 
waters  are  able  to  obtain  their  supply  of  oxygen,  for  the 
surface  of  the  ocean  is  in  constant  contact  with  the  great 
body  of  air.     In  the  case  of  the  animals  of  the  ocean  bottom, 


172  PHYSICAL   GEOGRAPHY, 

this  is  far  from  being  true  ;  and  yet  they  are  constantly 
supplying  to  the  water  a  certain  amount  of  carbonic  acid  gas 
which  in  the  course  of  time  would  tend  to  so  vitiate  the 
water  that  life  could  not  exist. 

This  is  one  of  the  strongest  arguments  in  favor  of  a  cir- 
culation of  the  waters  along  the  bottom  of  the  ocean,  from 
polar  to  tropical  regions.  There  must  be  some  supply  of 
oxygen  furnished  to  these  deep-sea  animals,  otherwise  they 
could  not  exist ;  and  there  is  no  other  supply  known  than 
that  which  may  be  brought  by  this  great  oceanic  circulation. 

Since  everything  points  to  the  conclusion  that  this  series 
of  ocean  movements  along  the  bottom  is  very  slow,  it  is  not 
unlikely  that  another  limitation  to  the  spread  of  deep-sea 
animals,  is  the  lack  of  abundant  oxygen.  For  if  there  is  not 
much  supplied  to  the  water,  there  cannot  be  much  taken  out. 
Therefore  the  existence  of  life  on  the  ocean  bottom,  appears 
to  depend  upon  several  conditions  which  are  more  or  less 
important ;  one  of  these  is  temperature,  another  is  food 
supply,  and  a  third  is  a  supply  of  oxygen. 


-•o*- 


REFERENCE     BOOKS. 

Williams. — The  Geography  op  the  Oceans.  Philip  &  Son,  London,  1881. 
16mo.  New  edition  in  the  press.  (An  accumulation  of  fact  and  purely 
descriptive  matter.) 

Shaler.  —  Sea  AND  Land.  Scribner,  New  York,  1894.  8vo.  |2.50.  (Much 
information  and  discussion,  particularly  with  relation  to  the  coast  line.) 

Thomson.  — The  Depths  of  the  Sea.  Macmillan  &  Co.,  New  York,  1873. 
8vo.  17.50.  (A  general  discussion  of  the  life  and  conditions  of  the  ocean 
depths.) 

Thomson.  —  The  Atlantic.  McDonough,  Albany,  N.Y.  8vo.  Vol.  L  and 
II.,  $3.00.  [Published  originally  by  Harper  Bros.]  (Very  full  account 
of  the  conditions  existing  on  the  ocean  bottom,  as  revealed  by  the  explora- 
tions of  the  British  ship  Challenger.) 


GENERAL   CHARACTEBISTICS   OF  THE  OCEAN.       173 

Reports  on  the  voyage  of  the  Challenger.  —  Narrative.  Vol.  I.,  Parts  I. 
and  II.  Eyre  and  Spottiswoode,  London,  1885.  4to.  £5  16s.  6d.  Pub- 
lished for  the  British  government.  See  also  Summary,  Vol.  I.  Price  80s. 
(The  best  and  latest  account  of  the  history  of  the  deep-sea  exploration. 
Contains  several  excellent  charts  of  the  ocean  bottom.) 

Agassiz.  —  Three  Cruises  op  the  Blake.  Houghton,  Mifflin  &  Co., 
Boston,  1888.  8vo.  Vol.  I.  and  II.,  ^8.00.  (The  most  recent  and  accurate 
description  of  the  depths  of  the  Atlantic,  particularly  of  the  Gulf  and 
Caribbean  region.) 

Wild. — Thalassa.  Marcus  Ward  &  Co.,  London,  1877.  8vo.  12s.  (Much 
on  depth,  temperature,  and  currents. ) 

Thoulet.  —  Oceanographie  (Statique).  Baudoin,  Paris,  1890.  8vo.  10  fr. 
(Much  of  importance  on  the  physical  questions  relating  to  the  ocean.) 

Sigsbee.  —  Deep-sea  Sounding  and  Dredging.  United  States  Coast  Survey, 
Washington,  1880.  (A  splendidly  illustrated  description  of  the  methods 
employed  in  deep-sea  exploration. ) 

Holder. —  Living  Lights.  Scribner,  New  York,  1887.  8vo.  ^1.75.  (A 
popular  description  of  phosphorescent  animals  on  the  land  and  in  the  sea. ) 

Murray  and  Renard.  —  Volume  on  Deep-sea  Deposits,  in  the  Challenger 
Reports.  Eyre  &  Spottiswoode,  London,  1891.  4to.  42s.  (A  very 
complete  discussion  of  deep-sea  deposits.     Beautifully  illustrated.) 

Moseley.  —  Notes  by  a  Naturalist.  Murray,  London,  1892.  8vo.  9s. 
(Narrative  based  upon  the  voyage  of  the  Challenger,  and  containing  much 
on  animal  distribution  and  peculiarities.) 

The  immense  mass  of  information  on  this  subject  accumulated  by  the 
Challenger  is  published  in  an  extensive  series  of  over  thirty  quarto  volumes. 
The  set  is  very  expensive ;  but  many  of  the  points  of  most  general  interest 
are  found  in  the  two  volumes  of  Narrative  and  the  Summary  referred 
to  above. 

The  Annual  Reports  of  the  U.  S.  Fish  Commission  also  contain  much  on 
deep-sea  exploration  ;  but  it  is  scattered,  and  mainly  found  in  the  earlier 
volumes,  which  are  now  difficult  to  obtain  free  of  cost. 


CHAPTER   X. 

OCEAN   WAVES   AND    CURRENTS. 

Wind  Waves. ^  —  As  a  result  of  friction  between  wind  and 
water,  the  ocean  surface  is  readily  started  in  motion  in  a 


Fig.  80. 
Ocean  waves.    Copyrighted,  1871,  by  Proctor  Bros.,  Gloucester,  Mass. 

series  of  wave-like  risings  and  fallings.  Normally  these 
wind  waves  are  swells,  with  alternate  ridge-like  troughs 
and  crests ;  but  where  broken  by  violent  winds,  they  may 

1  For  discussion  of  the  effect  of  waves  on  the  coast,  see  Chapter  XVIII. 

174 


OCEAN   WAVES  AND   CURBENTS. 


175 


be  cut  into  a  series  of  chops  or  angular  crests  (Fig.  80). 
The  water  movement  consists  of  oscillatory  risings  and  fall- 
ings of  water  particles,  while  the  waveform  passes  across  the 
water  in  the  direction  toward  which  the  wind  is  blowing. 
As  the  wave  passes  on,  a  floating  object  rises  and  falls  as  the 
troughs  and  crests  of  the  waves  pass  over  the  surface,  show- 
ing that  the  water  itself  is  not  in  horizontal  movement. 
In  realitv,  the  friction  of  the  air  does  drive  some  of  the  sur- 


Fig,   81. 
Breakers  on  the  coast. 


face  water  along,  and  therefore  if  a  body  could  float  entirely 
submerged  in  water,  so  as  to  be  out  of  the  direct  influence 
of  the  wind,  as  each  wave  passed  on,  it  would  continue  to 
rise  and  fall,  but  it  would  also  move  a  short  distance  in  the 
direction  toward  which  the  wave  was  moving. 

When  a  wave  approaches  the  shore,  its  form  and  behavior 
are  greatly  changed.  The  rising  and  falling  particles  of 
water  encouriter  the  bottom,  the  top  of  the  wave  combs  over, 


176  PHYSICAL   GEOGRAPHY. 

and  it  dashes  upon  the  coast  in  the  form  of  a  breaker 
(Fig.  81).  The  wave  is  such  a  shallow  movement  in  the 
water  that  it  is  readily  destroyed  upon  reaching  an  irregular 
coast.  Thus  in  harbors  or  bays,  the  violent  ocean  waves 
lose  their  force,  largely  because  of  friction  upon  the  shores 
and  bottom. 

A  very  slight  breeze  will  cause  a  series  of  wave-like  move- 
ments or  ripples ;  but  as  the  wind  continues,  and  its  force 
increases,  the  water  surface  may  be  thrown  into  a  series  of 
great  undulations.  The  water  is  so  mobile  that  these  wave 
movements  are  transmitted  for  great  distances,  and  they 
often  extend  far  beyond  the  place  of  origin.  One  may  see 
this  illustrated  upon  the  surface  of  almost  any  lake  over 
which  a  steamer  is  passing.  The  series  of  waves  started  by 
the  movement  of  the  steamer  through  the  water,  extend  out- 
ward for  miles  before  losing  their  form.  Upon  the  ocean  it 
is  not  uncommon  to  find  great  swells  or  rollers,  although  the 
sky  is  clear,  the  air  calm,  and  the  water  glassy,  —  their  origin 
generally  being  some  distant  storm. 

During  almost  all  times  of  day,  even  when  the  air  is  quiet, 
the  waves  beat  upon  exposed  coasts.  When  the  winds  are 
severe,  waves  often  rise  to  unusual  heights  and  beat  against 
the  coast  with  terrific  violence.  They  dash  against  the 
exposed  highlands,  sending  spray  into  the  air,  often  to  the 
height  of  two  or  three  hundred  feet ;  and  at  these  unusual 
times,  great  boulders  may  be  wrested  from  the  rocky  shore 
and  hurled  above  the  line  of  the  ordinary  ocean  surface. 
In  some  cases,  in  times  of  unusual  storms,  lighthouses  have 
been  washed  away.  Usually  the  effect  of  the  waves  is  con- 
fined to  that  part  of  the  coast  which  is  within  a  few  feet  of 
high-tide  mark.  But  during  these  unusual  storms,  the  action 
of  the  wind  waves  may  be  extended  a  number  of  feet  above 
this  point,  reaching  places  which  for  many  years  had  been 


OCEAN    WAVES  AND   CUBRENTS, 


177 


considered  safe  from  wave  attack.  During  a  storm,  a  few 
years  ago,  many  summer  cottages  on  the  sandy  coast  of  New 
Jersey  were  attacked  and  destroyed  by  the  waves,  and  a 
railroad  that  crosses  the  beach  was  torn  down  (Fig.  82). 

These  effects  of  waves  attract  our  attention  because  they 
are  unusual ;  but  the  every-day  action  of  the  wind  waves  is 
also  of  great  importance.      They  are   constantly  battering 


*  "r*        **rw"«*f^*  w  ^  *  iv**" 


"  \-7,  l'^'.^«^-^f?S^{  ^  >..-.,5  K<-^i«  .,, 


%     '      .\ 


Fig.  82. 
Effect  of  storm  waves  on  the  New  Jersey  coast. 

against  the  coast  and  tending  to  wear  it  away,  while  the 
wind-formed  currents  and  the  undertow  are  important  aids 
in  the  removal  of  the  loose  materials  thus  wrested  from  the 
shore.  In  many  places,  as  for  instance  in  Boston  Harbor,  it 
has  been  found  necessary  to  build  sea  walls  in  order  to  save 
from  destruction  some  of  the  exposed  islands  which  are  com- 
posed of  unconsolidated  gravel. 

N 


178  PHYSICAL    GEOGRAPHY, 

If  we  watch  the  rushing  of  the  waves  against  exposed 
coasts,  or  the  breaking  of  the  rollers  upon  the  sloping 
beaches,  we  are  able  to  form  some  conception  of  the  vast 
amount  of  destructive  work  that  these  oceanic  agents  may 
do  in  the  course  of  long  periods  of  time.  With  every  rush 
of  the  water  upon  the  beach,  pebbles  and  sand  are  dragged 
backward  and  forward;  and  this  constant  friction  of  one 
particle  upon  another,  in  the  course  of  time  will  cause  even 
the  hardest  rocks  to  wear  away.  In  the  course  of  a  few 
years  fragments  of  brick  or  glass  become  rounded,  so  that 
they  resemble  the  form  of  the  true  beach  pebbles ;  and  in  a 
year  or  two  a  brick  may  be  reduced  to  a  pebble  only  a  small 
fraction  of  the  size  of  the  original. 

Earthquake  Waves.  — When  an  earthquake  shock  disturbs 
the  waters  of  the  ocean,  a  great  wave  is  formed,  which 
extends  from  the  bottom  of  the  sea  to  the  surface,  and 
which  is  therefore  much  more  profound  in  its  effect  than  the 
shallow  wind  waves.  In  the  mid-ocean  these  earthquake 
waves  may  not  be  perceptible ;  but  as  they  reach  shallow 
coasts,  they  may  become  noticeable  as  their  elevation  is 
increased  in  the  shallowing  water.  Upon  reaching  coasts 
not  far  from  the  point  of  origin,  they  may  have  a  height  of 
from  50  to  100  feet,  which  gives  them  the  power  of  rushing 
upon  the  shore  to  a  much  greater  distance  than  ordinary 
waves  are  capable  of  reaching. 

During  some  earthquake  shocks,  the  water  wave  has 
extended  over  low  coasts  and  destroyed  scores  of  thousands 
of  lives.  Fortunately  this  form  of  ocean  disturbance  is 
rare,  and  it  is  a  type  of  wave  which  is  not  common  in  the 
Atlantic  Ocean.  Along  the  west  coast  of  South  America, 
and  on  the  Asiatic  coast,  where  earthquakes  and  volcanic 
eruptions  are  frequent,  the  earthquake  wave  assumes  very 
great  importance.     It  travels  at  a  rate  of  from  three  to  four 


OCEAN   WAVES  AND   CUBRENTS.  179 

hundred  miles  an  hour,  and  may  extend  for  a  distance  of  six 
or  seven  thousand  miles  from  the  place  of  origin;  but  at 
such  great  distances  it  has  so  lost  its  force  that  it  produces 
no  destructive  effect. 

Among  the  important  effects  of  these  rare  waves  is  the 
destruction  of  life  in  the  ocean.  An  explosion  of  dynamite 
in  water  will  kill  the  fishes  that  are  exposed  to  the  shock; 
and  near  its  source,  the  earthquake  wave  tends  to  cause  the 
same  kind  of  destruction. 

Storm  Waves.  — When  the  great  whirling  storms  of  cyclonic 
origin  (the  hurricanes  and  temperate  latitude  cyclones  de- 
scribed in  Chapter  V.)  pass  over  the  ocean,  the  spirally 
inblowing  winds  tend  to  heap  up  the  water  near  the  center 
of  the  storm.  In  the  center  the  air  pressure  is  less  than  on 
the  margins,  and  this  also  causes  the  water  near  the  center 
of  the  storm  to  rise.  Therefore  during  these  storms  there 
are  two  tendencies  to  the  production  of  unusually  high  water. 
When  the  storm  centers  pass  along  the  coast,  the  ocean  sur- 
face is  raised  to  a  height  often  as  great  as  six  or  eight  feet 
above  the  average ;  and  if  violent  wind  waves  accompany 
this  high  state  of  water,  their  destructiveness  along  the  shore 
becomes  greatly  increased. 

Any  strong  prevailing  wind  blowing  upon  the  coast,  tends 
to  raise  the  water  to  an  unusual  height.  During  the  passage 
of  waterspouts  over  a  portion  of  the  ocean,  there  is  raised 
a  cone-shaped  wave,  a  few  yards  across  the  base,  which,  on  a 
small  scale,  resembles  that  caused  by  the  passage  of  hurricanes. 

Ocean  Surface  Temperatures.  —  Latitude  is  the  most  impor- 
tant cause  for  differences  in  atmospheric  temperature,  and 
the  same  is  true  for  the  ocean.  Near  the  equator  the  oceanic 
waters  are  warmed,  while  near  the  poles  their  temperature 
remains  approximately  at  the  freezing-point  throughout  the 
year.     There  are  all  gradations  between  these  two  extremes 


180 


PHYSICAL   GEOGRAPHY, 


(Plates  15  and  27).  As  in  the  case  of  the  atmosphere,  this 
regularity  of  distribution  is  interfered  with  by  outside 
causes,  mainly  the  influence  of  land,  and  air  and  water 
movements  (Plates  15  and  27).  The  influence  of  the  ocean 
currents  is  shown  in  both  of  these  maps ;  and  they  also  show 

the  greater  regularity  of  the  ocean  surface 
isotherms  in  the  southern  hemisphere,  where 
there  is  little  land. 

Near  the  coast  the  temperatures  of  the 
ocean  surface  are  subjected  to  very  marked 
variations.  This  is  particularly  true  in  the 
temperate  zones,  where  the  difference  be- 
tween summer  and  winter  temperatures  is 
ver}^  great.  Thus,  on  the  New  England 
coast,  the  water  in  summer  is  warm  enough 
for  purposes  of  bathing,  Avhile  in  winter  it  is 
not  uncommonly  frozen  in  the  shallow  har- 
bors. Even  at  a  distance  of  a  number  of 
miles  from  the  shore,  this  variation  from 
summer  to  winter  is  quite  marked ;  but  in 
the  mid-ocean,  and  in  the  tropical  and  arctic 
zones,  the  summer  and  winter  temperatures 
are  very  nearly  the  same. 

In  the  ocean  there  is  a  vertical  change  in 
temperature.  Since  water  warms  very  slowly, 
the  effect  of  the  sun  extends  only  to  a  dis- 
tance of  a  few  score  of  feet,  even  in  the  trop- 
ics ;  and  below  this,  the  temperature  throughout  the  year  is 
practically  uniform,  while  it  rapidly  descends  until  the  cold 
waters  of  the  great  ocean  depths  are  encountered  (Fig. 
83).  Because  radiation  from  a  water  surface  is  a  slow 
process,  the  temperature  of  the  water  does  not  become 
rapidly  lowered  during  the  night.      Therefore  there  is  very 


Fig.  83. 

Diagram  to  show 
the  normal  de- 
scent of  temper- 
ature in  a  col- 
umn of  water 
in  the  ocean  at 
the  equator. 


S.D.S<rt<iii.N.T. 


ATIiAIN^TIC  OCEA]S^ 

SURFACE  TEMPERATURE  FOR  MARCH 

CENTIGRADE  SCALE  AND  FAHRENHEIT  SCALE 
^/ier  Kriimmil  from-Agaasiz 


Plate  15. 


182  PHYSICAL   GEOGBAPHY, 

little  reason  for  decided  changes  in  temperature,  either 
between  the  day  and  night,  or  between  the  seasons. 

Ocean  Currents :  Planetary  Circulation.  —  As  a  result  of 
differences  in  temperature  between  polar  and  tropical  re- 
gions, the  air  is  engaged  in  a  series  of  great  movements. 
There  are  many  reasons  for  believing  that  a  similar  circu- 
lation exists  in  the  ocean.  The  fact  of  the  difference  in 
temperature  suggests  the  probability  of  such  a  circulation, 
which  would  consist  of  a  rising  of  the  water  under  the 
equator,  a  surface  outflow  from  equatorial  to  polar  regions, 
and  then  a  downsinking  to  the  bottom,  from  which  there 
would  be  a  return  to  the  equatorial  regions  along  the  ocean 
bottom. 

That  this  theoretical  circulation  actually  exists,  is  sug- 
gested by  the  fact  that  the  bottom  of  the  sea  is  inhabited  by 
large  numbers  of  animals.  If  some  such  circulation  as  this 
did  not  exist,  it  would  be  difficult  to  account  for  the  supply 
of  oxygen  which  these  creatures  need  for  their  existence 
(pages  171  and  172). 

Such  planetary  circulation  seems  also  demanded  by  the 
temperature  conditions  of  the  ocean  bottom.  Unless  there 
has  been  a  downsinking  and  passage  of  Arctic  waters  over  the 
bottom  of  the  ocean,  we  cannot  explain  the  fact  that  temper- 
atures as  low  as  the  freezing-point  of  fresh  water  exist  over 
great  areas  in  the  depths  of  the  sea. 

A  circulation  is  also  suggested  by  the  peculiar  distribution 
of  temperature  on  the  ocean  bottom.  It  was  stated  on 
page  162  that  in  some  deep  parts  of  the  sea,  the  tempera- 
tures are  higher  than  in  other  portions  whose  depth  is  not  so 
great,  the  apparent  explanation  being  a  barrier  which  inter- 
feres with  the  passage  of  the  slowly  moving  water  over  the 
ocean  bottom.  Then  also,  under  the  equator,  a  temperature 
of  41°  is  encountered  at  a  depth  of  250  fathoms,  while  in  the 


--,'-  -^^~  ^^^>  ^^<> ^$>.<L—::^\i'<i,hi  i''hi''  0''/',/''///?^'/,//,//,.'/ ''//////. 


Face  page  183. 


'/'"Off; 


Warm.  CurrenU 


Cold  Currents  iL^c?-= 


^.D.Strroil.y.T. 


16. 


OCEAN   WAVES  AND   CUBBENTS.  188 

northern  hemisphere  this  temperature  is  reached  at  a 
depth  of  about  600  fathoms,  and  in  the  southern  hemi- 
sphere at  a  depth  of  about  400  fathoms.  This  indicates 
that  under  the  equator  the  cold  water  of  the  ocean  bottom 
is  rising. 

The  System  of  Ocean  Currents.  —  (Plate  16.)  We  know 
little  concerning  the  circulation  of  the  water  on  the  sea 
floor;  but  at  the  surface  there  are  certain  very  distinct 
movements,  to  which  the  name  ocean  currents  is  given.  In 
the  Atlantic  there  is  a  drift  of  surface  water  toward  the 
equatorial  portion  of  South  America.  This  slowly  moving 
surface  water  divides  against  the  triangular  coast  of  South 
America,  one  portion  passing  southward,  the  other  and 
larger  part  moving  northward,  still  as  a  slowly  moving 
drift. 

A  considerable  part  of  the  north-moving  drift  passes  into 
the  North  Atlantic  outside  of  the  West  Indies,  and  this  may 
be  called  the  North  Atlantic  Drift.  The  portion  which  enters 
the  Caribbean,  passes  through  the  Straits  of  Yucatan  into  the 
Gulf  of  Mexico,  where  a  part  of  it  circles  around,  and  finally 
emerges  past  Key  West  at  the  southern  end  of  Florida. 
Some  of  the  water  passes  northward  between  the  West 
Indies.  Therefore,  by  the  time  it  has  reached  the  latitude 
of  the  Carolinas  the  warm  current  of  the  North  Atlantic  is 
composed  of  several  parts. 

The  portion  which  emerges  from  the  Gulf  of  Mexico 
between  Cuba  and  Florida,  is  known  as  the  Gulf  Stream ; 
and  this  passes  northward  along  the  coast  as  a  very  percep- 
tible current,  on  the  seaward  side  of  which  is  a  portion  of 
the  Atlantic  drift,  which  did  not  enter  the  Gulf  of  Mexico. 
At  about  the  latitude  of  Cape  Hatteras,  the  Gulf  Stream  is 
turned  to  the  right  as  a  result  of  the  influence  of  the  earth's 
rotation,  and  it  then  passes  out  into  the  Atlantic  until  the 


184  PHYSICAL   GEOGRAPHY. 

European  shore  is  neared.^  A  branch  extends  northward 
into  the  Arctic,  while  a  part  returns  as  a  surface  current 
along  the  coast  of  Spain  and  Africa,  there  joining  the  equa- 
torial drift.  The  water  thus  eddies  around  in  the  North 
Atlantic,  moving  northward,  then  eastward,  southward,  and 
southwestward,  thus  establishing  a  complete  whirl.  Another 
important  current  in  the  North  Atlantic  is  the  cold  Labra- 
dor current  (see  p.  189). 

In  the  South  Atlantic  a  similar  whirl  of  water  is  caused ; 
but  this  is  distinctly  less  pronounced  than  that  of  the  North 
Atlantic,  and  there  is  no  current  so  marked  as  the  Gulf 
Stream.  Cold  water  from  the  Antarctic  extends  northward 
into  the  South  Atlantic. 

In  the  North  Pacific,  a  circulation  is  established  which 
very  closely  resembles  that  of  the  North  Atlantic.  A  broad 
equatorial  drift  passes  westward  toward  the  Asiatic  coast, 
then  becoming  in  part  a  north-moving  current,  it  proceeds 
as  a  very  distinct  stream,  in  many  respects  resembling  the 
Gulf  Stream.  This  is  known  under  the  name  of  the  Kuro 
Siwo,  or  better  as  the  Japanese  current.  It  passes  north- 
ward, is  turned  to  the  right,  then  moves  southeastward, 
bathing  the  western  coast  of  the  United  States,  then  curv- 
ing to  the  southwest,  it  joins  the  equatorial  drift.  Owing 
to  the  fact  that  land  practically  excludes  the  Arctic 
waters  from  the  North  Pacific,  there  is  no  distinct  Arctic 
current  in  this  ocean,  nor  is  the  Japanese  current  able  to 
extend  a  large  branch  into  the  Arctic.  Still  a  small  current 
of  cold  water  does  pass  through  Bering's  Straits  into  the 
North  Pacific. 

In  the  South  Pacific  and  Indian  oceans,  distinct  whirls  of 

1  Namerous  observations  on  the  movements  of  wrecks  and  floating  bottles 
have  given  us  much  information  concerning  this  current.  One  set  of  obser- 
vations upon  a  floating  wreck  is  shown  on  Plate  16. 


OCEAN    WAVES  AND   CURRENTS.  185 

water  are  produced,  which  more  nearly  resemble  the  whirl  of 
the  South  Atlantic  than  those  of  the  northern  oceans,  but 
which  nevertheless  are  better  developed  than  the  South 
Atlantic  system  of  currents.  Cold  currents  from  the 
Antarctic  extend  into  both  of  these  oceans. 

Thus  we  find  a  great  series  of  whirls  in  the  oceans,  one  on 
each  side  of  the  equator  in  each  ocean,  the  water  passing 
poleward  as  surface  currents  and  in  part  returning  to  com- 
plete the  whirl.  The  north-moving  system  is  better  devel- 
oped than  the  return  south-moving  currents,  and  the  system 
of  ocean  currents  is  much  better  developed  in  the  northern 
than  in  the  southern  oceans.  Particularly  is  this  true  of 
the  Atlantic.  In  any  explanation  of  oceanic  circulation 
these  facts  must  be  accounted  for. 

Cause  of  Ocean  Currents.  —  Although  there  is  evidence 
that  a  planetary  circulation  exists  in  the  ocean,  there  are 
many  reasons  for  doubting  whether  this  cause  is  sufficient  to 
explain  the  system  of  surface  currents  which  is  so  well  devel- 
oped. The  difference  in  temperature  does  not  seem  suffi- 
cient to  account  for  the  great  oceanic  whirls.  While  the  al- 
most constant  cold  of  the  Arctic  and  Antarctic  oceans  causes 
a  continual  descent  of  water,  it  cannot  be  said  that  the  heat 
at  the  equator  is  sufficient  to  so  expand  the  water  as  to 
cause  surface  currents  to  start  here. 

The  comparison  has  been  made  between  the  ocean  circu- 
lation and  that  of  the  air  ;  but  this  is  only  partly  warranted, 
for  the  air  is  warmed  from  below  by  contact  with  the  earth, 
and  when  warmed  this  lower  air  must  rise.  In  the  ocean, 
the  heat  of  the  sun  is  practically  confined  to  the  immediate 
surface ;  and  this  relatively  thin  layer  is  not  warmed  to  a 
very  high  degree,  so  that  its  expansion  would  not  seem  to 
be  sufficient  to  cause  a  flowing  away.  If  the  sun's  heat 
penetrated  to  the  ocean  bottom,  the  warming  of  the  lower 


186  PHYSICAL   GEOGRAPHY. 

layers  of  water  would  cause  sufficient  expansion  to  necessi- 
tate their  rise  to  the  surface ;  but  this  is  not  the  case. 

If  temperature  differences  account  for  ocean  currents,  the 
fact  of  the  greater  development  of  the  system  in  the  north- 
ern oceans  would  be  difficult  to  explain.  The  Antarctic  is 
practically  open  to  both  Atlantic  and  Pacific,  and  in  that 
hemisphere  there  is  an  excellent  opportunity  for  an  exchange 
of  polar  and  tropical  waters.  But  the  Arctic  is  almost  com- 
pletely shut  off  from  the  Pacific,  and  is  only  open  to  the 
Atlantic  through  narrow  and  rather  shallow  channels. 
Therefore,  in  the  hemisphere  where  the  least  favorable  con- 
ditions for  an  exchange  of  water  exist,  we  have  the  best 
developed  currents.  In  the  North  Pacific  there  seems  abso- 
lutely no  chance  for  the  general  passage  of  cold  northern 
waters  along  the  bottom  to  the  equator. 

This  and  other  reasons,  such  for  instance  as  the  presence 
of  cold  surface  currents  returning  from  the  Arctic,  cause 
great  doubt  as  to  the  validity  of  the  temperature  theory 
which  has  been  held  by  many  physicists.  It  seems  that  we 
are  forced  by  these  arguments  to  return  to  the  theory  which 
was  proposed  by  Benjamin  Franklin,  who  pointed  out  the 
fact  that  nearly  permanent  winds  are  blowing  toward  the 
equator  throughout  the  year.  These  trade  winds  necessarily 
drive  large  quant  it  i^  of  surface  water  before  them,  just  as 
the  winds  along  the  coast  will  cause  the  surface  water  to 
drift  before  them. 

Thus  the  water  is  being  heaped  up  in  equatorial  regions, 
and  this  seems  sufficient  to  account  for  the  great  whirls  ;  and 
there  are  many  facts  tending  toward  the  conclusion  that 
winds  are  the  prime  cause  for  these  currents.  Among  other 
things,  the  whirls  are  best  developed  in  the  northern  oceans. 
The  belt  of  calms,  which  separates  the  two  systems  of  trade 
winds,  is  mostly  north  of  the  equator,  and  the  northern  belt 


OCEAN   WAVES  AND   CUBRENTS.  187 

of  trade  winds  does  not  extend  south  of  the  equator  during 
the  northern  winter,  while  the  southern  belt  does  extend 
north  of  the  equator  during  the  northern  summer  (Plates  10 
and  11).  Therefore  there  is  a  greater  drift  of  water  in  the 
northern  hemisphere  than  in  the  southern.  Probably  the 
differences  in  temperature  aid  in  this  circulation ;  but  to 
this  cause  we  must  assign  a  secondary  importance. 

The  course  of  the  various  currents  is  in  part  determined 
by  the  outlines  of  the  continents.  If  there  were  no  conti- 
nents, the  effect  of  the  trade  winds  would  be  to  produce  a 
surface  drift,  which  would  tend  to  pass  around  the  earth, 
approximately  in  the  belt  of  calms.  The  north  and  south 
extension  of  land  in  the  form  of  continents,  interferes  with 
this  circulation,  and  causes  the  moving  water  to  pass  north- 
ward or  southward.  After  this  deflection,  there  is  a  con- 
tinued tendency  for  the  currents  to  turn,  under  the  influence 
of  the  earth's  rotation,  to  the  right  in  the  northern,  and  to 
the  left  in  the  southern  hemisphere.  These  two  facts  of 
continental  interference,  and  deflective  effect  of  the  earth's 
rotation,  are  mainly  responsible  for  the  paths  pursued  by  the 
currents,  and  for  the  great  system  of  whirls.  The  cold  sur- 
face currents  which  come  from  the  Arctic  and  Antarctic,  are 
probably  a  partial  return  of  the  warm  water  that  drifts  into 
these  zones.  • 

The  Gulf  Stream.  —  This,  which  is  the  best-known  of  ocean 
currents,  is  of  so  much  interest  to  us,  and  so  well  illustrates 
some  minor  phenomena  of  ocean  currents,  that  it  is  well 
to  examine  it  in  a  little  more  detail  than  has  been  done 
(Plate  17).  The  Gulf  Stream  proper  is  that  portion  of  the 
equatorial  drift  which  has  passed  through  the  Caribbean 
and  the  Gulf  of  Mexico.  During  its  passage  through  these 
warm  gulfs,  its  temperature  has  been  increased  so  that  it 
emerges  into  the  Atlantic  as  a  very  warm  current.     It  is 


188 


PHYSICAL   GEOGRAPHY. 


one  of  the  most  rapidly  moving  of  ocean  currents,  and  its 
rapidity  depends  upon  the  peculiar  effect  of  irregularities  in 


80' 


CHART  OF  THE 

gvijF  stream 

SHOWING  ITS  AXIS  AND  LIMITS 


^    70°', 
Boston  cx^   Cdpe'Cod,- 


New  Yoiit 


Washington 


NANTUCKET   t. 


<5       <,  ' 
Cape  HenryS/       n^ 

■^  JCape  Hatteras 

A       ^ 


Co 


Cape  Fear^ 


Charleston  ^--''    •• 


'J  f.-~.S-:;;V,- 


\ 


^^ 


pafie  \Canaoeral 


DRY 
T0RTUGA8  • 


Cape.'FloriUa  -^ 


t,. 


r 


^ 


8(0° 


'35^ 


B.D.S.rwu.W.r.         7t 


Plate  17. 


the  continental  outline.     Passing  into  the  Gulf  of  Mexico 
without  difficulty,  it  finds  itself  partially  enclosed.     The  one 


OCEAN    WAVES  AND   CURBENTS.  189 

place  of  easy  escape  is  in  the  narrow  passage  between  Key 
West  and  Cuba.  In  a  measure,  it  is  concentrated  here,  in 
a  manner  somewhat  analogous  to  the  concentration  of  water 
in  the  nozzle  of  a  hose. 

When  it  passes  through  the  channel  at  the  end  of  the 
Yucatan  peninsula,  its  velocity  is  only  about  ^  of  a  mile 
an  hour,  and  its  width  is  about  90  miles,  while  its  depth 
is  approximately  1000  fathoms.  When  it  emerges  past 
Key  West,  its  velocity  is  from  four  to  five  miles  an  hour,  its 
width  only  50  miles,  and  its  depth  about  350  fathoms.  If  it 
were  not  for  this  concentration,  the  Gulf  Stream  would  not  be 
such  an  important  factor  in  the  North  Atlantic.  Soon  after 
passing  through  this  narrow  channel  its  velocity  decreases ; 
and  by  the  time  it  has  reached  the  Banks  of  Newfoundland, 
its  rate  of  movement  is  less  than  half  that  which  it  had  on 
the  Florida  coast.  It  has  been  estimated  that  every  day,  the 
Gulf  Stream  carries  past  Florida  the  enormous  amount  of 
436,000,000,000,000  tons  of  water. 

The  Labrador  Current.  — The  Labrador  current  comes  from 
the  Arctic  between  Greenland  and  Labrador,  passing  down 
the  coast  of  Nova  Scotia  and  Ncav  England,  and  keeping 
close  to  the  coast,  because  of  the  influence  of  the  earth's 
rotation,  which  tends  to  make  it  curve  to  the  right.  It 
remains  as  a  surface  current  until  Massachusetts  Bay  is 
reached,  where  it  sinks  to  the  bottom,  owing  to  the  fact  that 
it  has  a  lower  temperature,  and  therefore  greater  density, 
than  the  surrounding  water.  But  its  influence  is  felt  upon 
the  continental  shelf  nearly  down  as  far  as  Cape  Hatteras. 

Effects  of  Ocean  Currents,  —  The  most  striking  effect  of 
currents  is  upon  the  temperature.  If  it  were  not  for  the 
existence  of  this  oceanic  circulation,  it  is  probable  that 
a  large  part  of  the  now  habitable  earth  Avould  be  ren- 
dered unfit  for  habitation.     Much  of   the  heat  received  in 


190  PHYSICAL   GEOGRAPHY. 

equatorial  regions  would  remain  there,  while  the  cold  of 
high  latitudes  would  increase,  and  the  temperature  in 
these  regions  would  be  reduced  to  very  low  degrees.  The 
equatorial  regions  would  be  much  hotter  than  at  present, 
while  the  high  temperate  and  arctic  belts  would  be  colder. 
The  immense  influence  of  these  currents  is  shown  by  the 
fact  that  in  the  latitude  of  Labrador,  —  a  bleak,  inhospitable 
land,  —  there  are  powerful  and  well -populated  countries  on 
the  other  side  of  the  ocean.  In  the  one  case,  cold  Arctic 
currents  flow  along  the  coast;  in  the  other,  the  climate  is 
tempered  by  the  warm  ocean  current. 

Ocean  streams  carry  vastly  more  heat  than  air  currents 
are  capable  of  doing.  Croll  has  estimated  that  the  Gulf 
Stream  alone  carries  as  much  heat  as  falls  upon  a  surface 
of  1,560,935  square  miles  at  the  equator.  He  says  that  this 
stream  carries  from  tropical  regions,  nearly  one-half  as  much 
heat  as  is  received  directly  from  the  sun  in  the  entire  Arctic. 

The  temperature  of  the  Pacific  coast  of  the  United  States 
is  greatly  moderated  by  the  warm  Japanese  current,  which 
carries  into  the  North  Pacific  a  large  store  of  heat;  and, 
both  on  this  coast  and  on  the  western  coast  of  Europe,  the 
warm  bodies  of  water  which  are  off-shore,  not  only  supply 
quantities  of  heat,  but  they  furnish  to  the  land  much  moist- 
ure which  is  condensed  in  the  form  of  rain. 

Franklin's  attention  was  called  to  the  Gulf  Stream  by 
reason  of  the  fact  that  sailing  vessels  made  their  voyage 
from  the  colonies  to  the  mother  country  in  a  shorter  time 
than  on  the  return.  Therefore,  in  some  cases,  currents  in 
the  ocean  are  an  aid  to  navigation  (see  Plate  16,  showing 
the  drifting  of  a  wreck  in  this  current).  Where  a  cold 
and  warm  current  are  side  by  side,  as  is  the  case  near 
Newfoundland,  fogs  are  abundant,  and  this  interferes  with 
navigation, 


OCEAN   WAVES  AND   CURBENTS.  191 

Since  the  currents  temper  the  ocean  waters  (see  Plate  15), 
they  tend  to  modify  the  conditions  upon  which  the  spread 
of  marine  animals  depends.  This  influence  is  particularly 
noticeable  among  the  coral  reefs.  The  warm  tropical  cur- 
rents carry  large  quantities  of  food  and  of  clear  water  to 
the  banks  upon  which  corals  are  developing ;  and  it  may  be 
said  that  in  this  indirect  way,  ocean  currents  are  an  impor- 
tant cause  for  coral  reefs.  Thus,  where  the  Gulf  Stream 
bathes  the  coast  of  Florida,  reefs  and  coral  keys  are  pro- 
duced ;  and  even  as  far  north  as  the  Bermudas,  coral  life 
is  possible  because  of  the  presence  of  the  warm  tropical 
current. 


-•o*- 


REFERENCE   BOOKS. 

In  many  of  the  books  referred  to  at  the  end  of  the  last  chapter,  there  is 
something  on  oceanic  movements.  See  particularly  Agassiz,  Wild,  and 
Thoulet. 

For  a  very  complete  discussion  of  ocean  currents,  see  Croll's  "  Climate 
and  Time,"  referred  to  at  the  end  of  Chapter  VII. 

Maury. — The  Physical  Geography  of  the  Sea.  (There  are  many  edi- 
tions of  this  book,  most  of  them,  and  the  best,  being  out  of  print  and 
obtainable  only  in  the  second-hand  condition.) 

Pillsbury.  —  The  Gulf  Stream,  "  U.  S.  Coast  Survey  Annual  Report  for 
1890,"  Appendix  10.  Washington,  1891.  Issued  by  the  survey  in  sepa- 
rate form.  (The  most  complete  discussion  of  the  Gulf  Stream  which  has 
been  printed.) 

Berghaus  Atlas,  volume  on  Hydrography.  Justus  Perthes,  Gotha, 
Germany,  1891.  15m.  (Contains  numerous  excellent  charts  of  currents, 
ocean  temperatures,  etc.) 


CHAPTER   XI. 

TIDES. 

Nature  of  the  Tidal  Wave.  —  Each  day  the  ocean  surface 
is  disturbed  by  two  waves  which  pass  about  the  earth  with 
great  rapidity  (fully  500  miles  an  hour  in  the  Atlantic),  and 
affect  the  entire  ocean,  from  surface  to  bottom.  The  actual 
height  of  the  tidal  wave  is  very  slight,  and  sailing  vessels  in 
the  mid-ocean  are  never  aware  of  its  existence.  When  it 
approaches  the  shore,  the  wave  is  subjected  to  a  variety  of 
complex  changes,  which  make  it  an  important  feature  of  the 
ocean.  On  the  coast,  the  water  gradually  rises  or  flows,  and 
as  gradually  falls  or  ebbs;  and  this  is  repeated,  with  marked 
regularity,  approximately  twice  each  day. 

Cause  of  Tides.  —  In  origin,  the  tide  is  directly  associated 
with  the  effect  of  the  moon  and  sun  upon  the  earth.  All 
bodies  in  space  are  engaged  in  a  mutual  attraction  which  we 
know  as  gravitation ;  and  the  effect  of  this  gravitative  at- 
traction is  proportional  to  the  product  of  the  masses,  and 
inversely  proportional  to  the  square  of  the  distance.  Every 
member  of  the  solar  system  is  exerting  an  attraction  upon 
the  earth.  Since  the  attraction  varies  with  the  mass,  such 
a  large  body  as  the  sun  would  produce  a  great  effect  if  its 
distance  were  not  so  great.  The  moon,  although  relatively 
small,  is  so  near  that  its  influence  upon  the  earth  is  much 
greater  than  that  of  the  larger  and  more  distant  sun. 

Leaving  the  sun  out  of  the  question  for  a  time,  let  us  see 
what  effect  the  attraction  of  the  moon  will  have  upon  the 

192 


TIDES.  193 

earth.  If  the  earth  were  all  liquid,  the  attractive  action  of 
the  moon  would  tend  to  destroy  the  sphere  and  change  it  to 
an  ellipse.  The  ellipse  would  project  toward  the  moon,  for 
that  portion  of  the  earth's  surface  which  was  nearest  the 
moon  would  be  most  attracted.  Since  the  rotation  of  the 
earth  causes  the  moon  to  appear  to  pass  through  the  heav- 
ens, this  ellipse  would  constantly  change  in  position,  with 
its  axis  always  pointing  toward  the  moon.  Therefore  the 
liquid  sphere  would  be  thrown  into  a  series  of  waves,  one 
crest  being  beneath  the  moon,  while  the  other  crest  was  on 
the  opposite  side  of  the  earth  farthest  from  the  moon. 

This  is  approximately  what  happens  in  the  liquid  ocean. 
The  surface  of  this  partial  liquid  covering  is  disturbed  by 
the  gravitative  attraction  of  the  moon,  and  a  wave  is  thus 
produced  beneath  it,  while  one  is  also  formed  on  the  opposite 
side  of  the  earth.  As  the  moon  appears  to  pass  around  the 
earth,  these  two  waves  also  move.  They  are  much  disturbed 
by  the  irregularity  of  the  continents,  and  their  movements 
are  rendered  very  complex  as  a  result  of  this  influence. 
They  are  not  able  to  remain  directly  beneath  the  moon,  but 
lag  behind  and  follow  it,  instead  of  passing  around  the  earth 
with  it. 

In  a  similar  manner  the  sun  causes  two  waves ;  and  these 
combine  with  those  produced  by  the  moon  to  produce  the 
tidal  wave.  Since  the  movements  of  the  sun,  the  moon,  and 
the  earth  are  very  irregular,  there  are  many  complexities 
introduced  into  the  tidal  movement.  In  the  latter  part  of 
the  chapter  some  of  these  irregularities  are  considered,  and 
their  cause  pointed  out. 

Effect  of  the  Land.  —  The  tide  waves  tend  to  pass  about 
the  earth  from  east  to  west,  following  the  direction  of  the 
path  of  the  moon  through  the  heavens.  This  west-moving 
tide  wave  is  much  better  developed  in  the  great  expanse  of 


194  PHYSICAL   GEOGRAPHY. 

water  in  the  southern  hemisphere,  than  in  the  northern. 
The  continents  interfere  with  the  movement  of  the  wave, 
and  in  some  cases  successfully  check  it.  When  the  wave 
enters  the  Atlantic,  its  direction  is  changed  from  west  to 
north,  and  soon  the  wave  is  so  changed  that  it  advances 
more  rapidly  in  the  middle  part  of  the  ocean  than  on  the 
margins  (Plate  18).  This  is  due  to  the  effect  of  the  shallow 
waters  near  the  continents.  The  wave  is  retarded  near  the 
shore  and  advances  more  rapidly  in  the  central  portion  of 
the  ocean.  As  a  result  of  this,  the  crest  of  a  wave  may  have 
reached  the  latitude  of  Newfoundland  at  the  same  time  that 
the  margins  are  affecting  the  coast  of  northern  Africa  and 
the  West  Indies. 

In  a  similar  way  this  effect  of  friction  is  also  shown  in  the 
bays  and  larger  estuaries.  Thus  as  the  wave  passes  up  the 
Bay  of  Fundy,  friction  with  the  shore  causes  it  to  be  re- 
tarded, while  in  the  central  part  of  the  bay  the  wave  advances 
more  rapidly. 

Nowhere  can  this  effect  of  the  land  be  better  illustrated 
than  in  the  vicinity  of  the  British  Isles  (Plate  19).  The 
wave  passes  up  the  Atlantic,  and  without  serious  inter- 
ference moves  to  the  northern  extremity  of  Ireland  and  Scot- 
land, while  the  same  wave  has  advanced  to  the  southern 
coast  of  England,  and  begun  to  pass  into  the  English  Chan- 
nel. The  shallowness  of  the  water  in  this  region  prevents 
the  rapid  movement  of  the  wave ;  and  by  the  time  it  has 
passed  through  the  English  Channel,  the  part  that  went  out- 
side of  the  British  Isles  has  gone  entirely  around  the  islands, 
and  entered  the  North  Sea,  where  the  two  parts  of  the  same 
wave  meet  (Plate  19). 

On  the  American  coast  a  similar  influence  of  the  land  is 
noticed  in  the  approaches  to  New  York  Harbor.  The  tidal 
wave  passes  readily  up  the  bay  toward  New  York,  while  the 


^<n'?'^^ 


><J55to' 


Arctic  Circle 


Face  page  194. 


Diagrammatic  representation  of  the  advance  o] 


5  18. 

tidal  waves.    Figures  refer  to  noon  and  midnight. 


Plate  19. 

Diagrammatic  representation  of  the  tidal  wave  near  the  British  Isles.    Figures 

refer  to  hours  of  the  day. 


196 


PHYSICAL   GEOGRAPHY. 


same  wave  goes  around  the  eastern  end  of  Long  Island  into 
Long  Island  Sound  (Fig.  84).     Here  its  rate  of  motion  is 

retarded,  the  dis- 
tance traveled  is 
greater,  and  at 
Hell  Gate  Chan- 
nel the  two  parts 
of  the  same  wave 
arrive  at  entirely- 
different  times 
(Fig.  85).  This 
Fig.  84.  is  one  of  the  rea- 

Diagram  to  show  path  pursued  by  the  tides  on  the  two    sons   for   the  vio- 

Figures  represent  height  of  tides    ^^^^^    currents    at 

Hell  Gate.   ' 

Aside  from  this  influence  of  coastal  irregularities   upon 
the  time  of  approach   of 


sides  of  Hell  Gate, 
at  different  places. 


Ill     IV 


HOURS  AFTER  TRANSIT 
VI     VII    VIII     IX       X       XI       0 


the  tidal  wave,  these  pe- 
culiarities also  influence 
the  height  to  which  the 
tide  rises.  The  normal 
tidal  rise,  as  observed 
in  mid-ocean  and  on  ex- 
posed coasts,  is  only  one 
or  two  feet.  Along  the 
eastern  coast  of  America, 
we  find  the  tide  rising 
in  one  place  only  two  or 
three  feet,  in  other  places 
10  or  20  feet,  and  in  the 
Bay  of  Fundy  often  50  or 
60  feet.  These  irregu- 
larities are  due  to  the  influence  of  the  coastal  outline. 


I    II    III 


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Fig.  85. 
Diagram  to  show  time  of  arrival  and  height 
reached  by  the  tides  on  the  two  sides  of 
Hell  Gate. 


TIDES.  197 

There  are  two  ways  in  which  the  tide  may  be  almost 
entirely  destroyed.  It  is  a  familiar  fact,  that  if  tAvo  waves 
meet  trough  to  crest,  they  extinguish  one  another.  It  is 
believed  that  the  two  tidal  waves  which  meet  in  the  North 
Sea  (Plate  19),  actually  come  together  in  this  way. 

When  the  tide  enters  a  large  body  of  water  through  a 
narrow  inlet,  the  tidal  rise  is  almost  entirely  destroyed,  as  is 
very  well  illustrated  in  the  Mediterranean.  Outside  of  the 
Straits  of  Gibraltar,  on  the  coast  of  Spain,  the  height  of  the 
tide  is  from  five  to  six  feet.  The  wave  enters  the  Mediter- 
ranean through  this  narrow  inlet,  then  expands,  and  con- 
sequently loses  in  height,  until  almost  no  tide  is  left.  In 
portions  of  the  Mediterranean  there  are  slight  tides,  but 
these  appear  to  depend  in  part  upon  another  cause. 

The  opposite  effect  of  increase  in  height  of  the  tide  is  by  far 
the  most  common  influence  of  coast  irregularities.  When, 
instead  of  entering  a  large  body  of  water  through  a  narrow 
inlet,  the  tidal  wave  passes  into  a  narrowing  bay  through  a 
broad  mouth,  the  effect  of  the  converging  shores  is  to  pile 
up  the  wave,  and  therefore  to  increase  the  height  of  the  tide. 
This  is  the  cause  for  the  very  high  tides  of  the  Bay  of  Fundy, 
and  many  other  V-shaped  bays  and  estuaries.  It  is  well 
illustrated  in  Massachusetts  Bay,  where  the  rise  of  the  tide 
is  between  8  and  12  feet. 

As  a  result  of  the  influence  of  coast  irregularities,  some 
peculiar  tidal  effects  are  produced.  In  two  neighboring,  and 
possibly  connected  bays,  the  height  of  the  tide  may  be  quite 
different.  This  is  the  case  in  Vineyard  Sound  and  Buzzard's 
Bay,  on  the  south  coast  of  Massachusetts,  where,  in  the  lat- 
ter, the  tide  rises  one  or  two  feet  higher  than  in  Vineyard 
Sound,  which  is  open  on  both  ends.  In  the  channels  which 
connect  these  two  bays,  violent  currents  are  produced; 
and  this  whole  region,  between  the  Elizabeth  Islands   and 


198  PHYSICAL   GEOGRAPHY. 

Nantucket,  is  one  of  relatively  rapid  tidal  currents.  The 
rapid  currents  in  the  straits  between  two  such  bodies  of 
water,  may  be  called  tidal  races. 

A  tidal  race  is  produced  at  Hell  Gate,  near  New  York 
City,  mainly  because  the  tide  rises  higher  in  Long  Inland 
Sound  than  it  does  in  the  bay  of  New  York  Harbor  (Figs.  84 
and  85).  The  very  rapid  currents  in  this  shallow  strait,  are 
in  part  due  to  this  cause,  and  in  part  to  the  fact  that  the 
time  of  high  tide  is  different  on  the  two  sides  of  Hell  Gate. 
Similar  tidal  races  occur  on  many  parts  of  the  irregular 
northern  shore,  and  at  times  currents  are  produced  which 
are  as  violent  as  rapidly  moving  streams.  In  some  cases  it 
is  impossible  to  row  a  boat  against  the  current. 

In  a  rapidly  narrowing  bay,  particularly  at  the  mouth  of 
a  river,  the  rising  tide  is  sometimes  transformed  to  a  wave, 
which  in  form  resembles  the  wind  wave ;  and  there  is  an 
advancing  wall  of  water,  instead  of  the  gradual,  almost 
imperceptible  rising  of  the  ocean  surface,  which  is  the  normal 
form  of  the  incoming  tide.  To  this  peculiar  phenomenon 
the  name  tidal  bore  is  given.  This  wave  is  produced  in  the 
Amazon,  the  Severn,  the  Seine,  and  many  other  rivers. 

Other  Causes  for  Variation  in  Tidal  Height.  —  At  any  given 
point  on  the  coast,  the  height  of  the  tide  is  liable  to  vary  from 
time  to  time.  This  variation  may  be  of  an  irregular  nature, 
due  to  the  effect  of  winds  upon  the  surface  of  the  water. 
Sometimes,  when  strong  winds  blow  upon  the  coast,  the 
height  of  the  tide  may  be  increased  several  feet.  A  mere 
change  in  the  pressure  of  the  air  also  appears  to  cause  fluc- 
tuations in  the  surface  of  the  sea ;  and  upon  lakes,  these 
causes  produce  fluctuations  in  level  which  are  often  of  quite 
noticeable  size.  In  the  Swiss  lakes  these  irregular  variations 
in  the  level  of  the  water  are  known  as  seiches,  and  they  are 
also  found  upon  the  Great  Lakes. 


TIDES. 


199 


The  main  variations  in 
the  height  of  tide  depend 
upon  astronomical  causes. 
Since  the  tide  is  the  com- 
bination of  two  waves,  one 
produced  by  the  sun,  and 
the  other  by  the  moon,  the 
height  of  the  tide  naturally 
varies  as  the  position  of 
these  bodies  in  the  heavens 
changes.  During  new 
moon,  the  sun  and  moon 
are  nearly  in  the  same  line, 
and  they  therefore  pull 
approximately  along  the 
same  line,  so  that  the 
unusually  high  tide  then 
produced  (known  as  spring 
tide)^  is  the  result  of  a 
combination  of  the  two 
waves.  During  full  moon, 
the  sun  and  moon  are 
again  in  line,  one  on  either 
side  of  the  earth,  and  then 
the  two  waves  again  tend 
to  combine.  Therefore 
every  month  there  are  two 
sets  of  rather  strong  or 
high  tides  (Fig.  86). 
Between  new  and  full 
moon,  —  that  is,  during 
the  first  and  third  quar- 
ters,—  the  sun  and  moon 


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200 


PHYSICAL   GEOGRAPHY. 


are  pulling  upon  the  earth  at  an  angle,  and  then  unusually 
weak  or  low  tides,  known  as  neap  tides,  are  produced. 

In  the  movement  of  the  moon  around  the  earth,  it  follows 
a  path  which  is  quite  elliptical.  Therefore,  since  the  earth 
is  at  one  of  the  foci,  there  is  a  time  during  every  lunar 
month  when  the  moon  is  much  nearer  the  earth  than  when 
it  is  in  the  opposite  part  of  its  elliptical  path.  When  the 
moon  is  nearest  to  the  earth,  it  is  said  to  be  in  perigee,  and 
when  farthest  from  the  earth,  in  apogee.  Since  the  tide- 
producing  force  varies  greatly  with  the  distance,  this  differ- 
ence in  lunar  distance  produces  a  very  marked  effect  upon 


1893 

1894 

Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sep. 

Oct. 

Nov. 

Dec. 

Jan. 

Feb. 

Mar 

Apr. 

May 'June 

July 

Aug, 

Sep. 

Oct. 

Nov. 

Dec. 

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Fig.  87. 

Height  of  the  high  tide  at  Eastport,  Maine,  in  1893  and  1894.    Cross  indicates 
apogee ;  dot,  perigee ;  shaded  circle,  new  moon ;  plain  circle,  full  moon. 


the  strength  of  the  tide.  Thus  it  will  be  seen  that  when  the 
new  or  full  moon  comes  during  perigee,  a  high  range  of  tides 
will  result,  because  then  the  moon  is  both  nearer  to  the 
earth,  and  its  tide  is  combined  with  that  caused  by  the  sun. 
If  new  or  full  moon  occurs  during  apogee,  the  tides  are  not  so 
strong,  because  then,  although  the  solar  and  lunar  tides  are 
combined,  the  moon  is  farther  from  the  earth.  When  apogee 
occurs  at  one  of  the  quarters,  the  tides  are  unusually  low ; 
and  when  perigee  occurs  at  this  time,  the  effect  of  opposition 
of  sun  and  moon  is  partly  counterbalanced  (Figs.  86  and  87). 
Other  movements  of  the  earth,  sun,  and  moon  introduce 
complexities  in  the  tidal  rise  and  fall.     For  instance,  in  some 


TIDES.  201 

seasons  the  sun  is  nearer  the  earth  than  at  others,  and  at 
times  the  moon  is  more  nearly  over  the  equator  than  at  other 
times ;  and  all  of  these  variations  produce  an  effect  upon 
the  tide.  One  notices  in  Fig.  86  that  the  two  tides  for  any 
single  day  are  different  in  height ;  and  this  difference  varies 
in  various  parts  of  the  month.  All  of  these  irregularities 
are  capable  of  explanation,  and  are  well  understood ;  but  it 
will  be  impossible  to  give  the  space  for  their  consideration 
in  this  book. 

One  point  is  worth  special  attention,  —  that  the  time  be- 
tween two  high  tides  is  not  exactly  a  half  day,  because  the 
tide  wave  travels  on  lunar,  and  not  on  solar  time.  The  tide 
rises  once  every  12  h.  25  m.,  so  that  each  day  the  high  tide 
is  about  50  minutes  later  than  the  corresponding  tide  for 
the  previous  day. 

Effects  of  Tides.  —  Along  irregular  coasts,  where  the  tide 
rises  to  a  heiofht  of  several  feet,  and  where  tidal  currents  are 
produced,  the  influence  of  the  rise  and  fall  of  the  tide  is  of 
considerable  importance  in  navigation ;  and  before  sailing, 
many  vessels  wait  until  a  favorable  time  of  tide.  This  is 
particularly  the  case  when  ships  are  about  to  sail  from  ports 
that  are  obstructed  by  bars,  which  at  low  tide  are  so  near 
the  surface  that  some  ships  are  unable  to  pass  over  them. 
This  is  the  case  in  many  harbors  of  the  world. 

Because  they  are  much  less  powerful,  tidal  currents  are 
not  wearing  the  coast  in  the  way  that  wind  waves  are  ;  but 
they  are  doing  a  certain  work  in  changing  the  form  of  the 
coast,  mainly  as  the  result  of  transportation  of  fragments 
derived  from  the  rocks  by  the  beating  of  the  waves.  On 
some  coasts,  as  for  instance  in  the  English  Channel,  and 
near  Nantucket,  the  action  of  the  currents,  by  the  constant 
movement  of  the  sands,  is  sufficient  to  cause  frequent  changes 
in  the  depth  of  the  water. 


202 


PH:fSICAL   GEOGRAPHY. 


Where  the  tide  rises  in  the  mouths  of  rivers  or  in  estua- 
ries, as  in  Chesapeake  and  Delaware  bays,  the  rise  of  the 
tide  checks  the  river  water,  and  causes  it  to  deposit  what 
sediment  it  is  carrying,  so  that  this  effect  is  also  important 
in  modifying  the  bottom  of  these  bays  (Fig.  88).  Many 
harbors  are  being  filled  by  this  means,  and  millions  of  dollars 
are  every  year  expended  in  attempting  to  remove  the  mud 
and  sand  deposited  by  this  tidal  action. 


Fig.  88. 

Low  tide  in  Basin  of  Minas,  Nova  Scotia.    An  extensive  mud  flat,  submerged  at 
high  tide.     (Copyright,  1890,  by  S.  R.  Stoddard,  Glens  FaUs,  N.Y.) 


The  rise  and  fall  of  the  tides  is  a  great  force  in  the  ocean 
(Fig.  89),  constantly  acting,  and  capable  of  doing  a  great 
amount  of  work,  which  man  may  sometime  find  it  possible 
to  utilize.  Already,  in  some  places,  the  rising  and  falling 
tides  are  employed  for  local  purposes  of  water  power.  On 
the  New  England  and  Canadian  coast,  the  rising  tide  is 
allowed  to  freely  enter  some  broad,  bay-like  expansion  of 
the  coast,  from  which  it  is  prevented  from  escaping  by  means 


TIDES. 


203 


of  gates  that  automatically  close  as  the  tide  begins  to 
fall.  There  is  then  pro- 
duced a  rather  large  pond,  '  --^ 
several  feet  above  the  low- 
water  mark ;  and  from  this, 
water  may  be  led  upon  a 
wheel,  and  then  made  to 
serve  for  mill  purposes. 
There  are  numerous  grist 
mills  along  the  coast  which 
are  run  by  tide -water 
poAver.  They  can  be  used 
only    a    few    hours    every                           Fig.  89. 

day,  but  it  is  a  very  inex-    Coast  of  Cape  Ann,  Mass.    To  show  tidal 

rnr^     -    X.  I'ise  and  fall.   The  dark-colored  areas  are 

pensive  power.     The  mtro-      ^^^^^^d  by  the  high  tide. 

duction  of  electricity  for  so 

many  uses,  may  make  it  possible  to  employ  this  vast  force 

much  more  commonly  than  has  been  done. 


-»<>♦- 


REFERENCE     BOOKS,  i 

Thomson.  —  Popular  Lectures  and  Addresses.  Vol.  III.,  Lecture  on 
the  Tides.  Macmillan  &  Co.,  New  York,  1891.  12mo.  $2.00.  (A  par- 
tial statement  of  the  tidal  theory). 

See  article  on  tides  in  Encyclopedia  Britannica. 

For  data  upon  time  and  height  of  tides,  see  Tide  Tables  for  the  Atlantic 
Coast,  U.  S.  Coast  Survey,  Washington,  D.C.  $0.25.  Published  an- 
nually.    There  is  also  a  similar  set  of  tables  for  the  Pacific  coast. 

^  The  subject  of  tides  is  difficult  to  present  clearly  in  non-mathematical 
terms,  and  hence  the  general  literature  is  quite  barren  upon  the  subject.  By 
far  the  most  that  has  been  written  upon  the  subject,  is  scattered  through  the 
proceedings  of  scientific  societies  and  the  magazines. 


Part   III. 


THE  LAND. 


CHAPTER    XII. 

THE   CRUST   OF   THE   EARTH. 

Interior  Condition.  —  Some  wells  and  mines  have  extended 
to  a  depth  of  over  a  mile  from  the  surface,  and  in  every  case 
it  is  found  that  the  temperature  increases  as  the  depth 
becomes  greater.  While  this  increase  is  not  regular,  on  the 
average  it  is  about  1°  for  every  50  or  60  feet  of  descent.  If 
this  increase  continues,  as  it  probably  does,  the  temperature 
at  the  depth  of  a  score  of  miles,  is  sufficiently  high  to  melt 
most  rocks  under  the  conditions  existing  at  the  surface. 
In  various  parts  of  the  earth,  molten  rock  reaches  the  surface 
through  volcanio  vents ;  and  there  are  other  indications  that 
high  temperatures  exist  within  the  earth. 

Until  within  a  few  years,  it  was  believed  that  beneath 
a  crust  of  comparative  thinness,  the  earth  was  in  a  molten 
condition,  and  that  the  solid  crust,  or  rind,  rested  upon  this 
liquid.  In  speaking  of  the  outside  of  the  earth,  we  still 
use  the  term  crust,  although  it  is  no  longer  believed  that 
the  interior  is  molten.  Many  facts,  some  astronomical, 
others  geological,  have  caused  the  abandonment  of  the  theory 
of  a  molten  interior  ;  and  it  is  now  believed,  that  although 
at  depths  only  a  few  miles  from  the  surface,  the  temperature 
is  high  enough  to  melt  rocks,  they  are  prevented  from 
becoming  molten  by  the  great  pressure  of  the  solid  strata  of 
the  crust.  This  energy  is  constantly  passing  from  the  interior 
to  the  surface,  where  it  is  radiated  into  space ;  and  this 
constant  loss  of  heat  causes  a  loss  of  bulk  through  con- 

205 


206  PHYSICAL    GEOGRAPHY. 

traction.  The  cold  outside  does  not  shrink ;  but  as  the 
interior  loses  in  size,  this  crust  becomes  wrinkled,  in  a  man- 
ner which  may  be  compared  with  the  wrinkling  of  the  skin 
of  an  apple  which  is  drying. 

Movements  of  the  Crust.  —  There  are  many  proofs  that 
the  crust  of  the  earth  is  in  movement.  Usually  these 
movements  are  so  slow  that  they  can  be  detected  only 
after  long  intervals  of  time ;  but  sometimes  rapid  changes 
have  actually  been  witnessed.  The  proofs  of  these  earth 
movements  may  be  said  to  be  of  two  kinds,  historical  and 
geological.  While  the  historical  proofs  may  perhaps  appear 
to  be  most  conclusive,  they  are  in  reality  much  less  impor- 
tant than  those  of  a  geological  nature. 

In  several  places  the  land  has  been  known  to  move  during 
earthquake  shocks,  and  to  remain  either  higher  or  loAver 
than  before  the  shock.  As  one  instance  of  this,  we  may  refer 
to  the  earthquake  of  1822,  during  which  the  whole  coast  of 
central  Chili  was  raised  from  three  to  four  feet.  In  other 
shocks  on  the  same  coast,  the  land  has  been  permanently 
elevated,  and  there  is  abundant  evidence  that  the  land  of 
this  coast  is  now  steadily  rising.  Near  Vesuvius,  in  Italy, 
there  are  columns  of  a  temple  which  were  built  above  the 
level  of  the  ocean,  and  are  now  above  it,  but  which  at  one 
time  were  submerged ;  and  they  therefore  register  two  move- 
ments of  the  land.  On  the  coast  of  Sweden,  it  was  believed 
that  the  land  was  slowly  moving,  but  so  slowly  that  without 
careful  measurements  it  could  not  actually  be  proven.  In 
order  to  thoroughly  test  the  matter,  marks  were  made  at 
the  water  surface,  and  after  a  number  of  years  examined, 
when  it  was  found  that  there  were  movements  over  an  area 
of  200  miles  in  extent.  North  of  Stockholm  the  rate  of 
elevation  is  as  much  as  two  or  three  feet  a  century. 

Of  geological  evidence,  perhaps  the  best  is  that  of  fossils, 


THE  CBUST  OF  THE  EARTH.  207 

which  have  attracted  attention  from  the  very  earliest  times. 
Remains  of  animals  that  must  have  dwelt  in  the  sea,  are 
found  in  many  of  the  rocks  of  all  continents,  and  at  all 
elevations,  even  on  the  highest  mountains.  The  rocks 
themselves  are  evidence  of  elevation,  for  in  many  cases 
they  are  of  kinds  which  we  know  must  have  been  formed 
in  the  sea. 

Along  the  coast  lines,  in  many  parts  of  the  earth,  beaches 
and  other  features  of  the  seacoast  are  found  at  a  distance 
above  the  present  sea  level ;  and  tree  trunks  which  we  know 
must  have  grown  on  the  land,  are  in  some  cases  below  the 
low-tide  mark.  The  shore  lines  of  lakes  which  once  existed 
in  the  interior,  but  have  now  disappeared,  also  give  evidence 
of  land  movement.  Since  they  were  formed  on  the  margin 
of  a  level  body  of  water,  they  must  have  been  horizontal ; 
but  in  some  cases  these  ancient  shore  lines  are  no  longer 
horizontal.  Other  evidences  might  be  brought  forward  in 
proof  of  a  change  in  the  relation  between  the  sea  level  and 
the  land. 

It  may  be  asked  whether  this  is  proof  of  changes  in  the 
level  of  the  sea,  or  of  land  movement.  While  there  is  reason 
to  believe  that  there  have  been  changes  in  the  sea  level,  the 
evidence  is  conclusive  that  the  greater  number  of  these 
changes  in  relation  of  land  to  sea,  are  due  to  actual  move- 
ments of  the  land.  Without  entering  into  this  subject  in 
detail,  it  may  be  stated,  that  the  most  conclusive  evidence 
that  this  change  is  due  to  land  movement,  is  the  fact  that 
many  of  the  rocks,  which  we  know  were  formed  as  nearly 
horizontal  layers  in  the  ocean,  are  now  found  in  mountains 
in  a  folded  and  often  broken  condition. 

Disturbance  of  the  Rocks.  —  In  many  cases,  the  rocks 
that  have  been  raised  from  the  sea,  to  form  a  part  of  the 
continent,  are  still  in  nearly  horizontal  positions  (Figs.  90 


208 


PHYSICAL   GEOGRAPHY. 


,#^#»<**^r^*- 


.X 


Fig.  90. 
Horizontal  rocks  on  the  plains  of  Kansas. 


and  133  and  Plate 
28) .  They  have  been 
bodily  raised  with 
very  little  disturb- 
ance. In  mountains, 
and  less  prominently 
elsewhere,  the  rocks 
have  been  moved 
from  their  horizontal 
position,  and  caused 
to  assume  inclined 
attitudes,  which  are 
often  very  complex.  These  changes  commonly  assume  one 
of  two  forms,  either  (1)  fold- 
ing or  (2)  breaking,  which 
we  call  faulting. 

Even  the  most  brittle  of 
rocks  may  be  folded.  The 
cause  for  the  folding  usually 
acts  so  slowly,  and  the  rocks 
are  under  such  pressure  from 
above,  that  they  bend,  rather  than  break,  when  subjected 

to  a  strain  such  as  that  which 
comes  from  contraction  of  the 
interior.  A  simple  kind  of  fold 
is  that  known  as  the  monocline 
(Fig.  91),  where  the  rocks  are 
inclined  in  only  one  direction. 
When  they  are  bent  up  in  the 
form  of  an  arch,  the  folds  are 
known  as  anticlines  (Fig.  92), 
Pj^  02  ^^d    tl^^     corresponding     down 

Anticline.  fold,  is   known    as   the    syncline 


Fig.  91. 
A  monocline  fold. 


THE  CRUST  OF  THE  EARTH. 


209 


(Fig.  93).  These  may  be  no 
more  than  a  few  inches  across 
the  base,  or  they  may  have  a 
width  of  several  miles,  with  a 
length  of  perhaps  a  score  of 
miles. 

Among  mountains  there  is 
often  an  extremely  complex  sys- 
tem of  disturbances,  the  nature 
of    which    can    best    be  under- 


Fig.  93. 
Syncline. 

stood  by  an  examination 
of  the  accompanying  fig- 
ures. At  times  the  folds 
are  very  regular  (Fig.  94), 
but  usually  they  are  un- 
symmetrical  (Fig.  95). 
They  are  generally  ridge- 


FiG.  94. 

Photograph  of  an  anticline  near  Hancock, 
W.  Va. 

like,  and  in  the  direction  of  the 
ridge  they  gradually  lose  in  size 
and  finally  disappear  altogether. 
The  direction  in  which  these  rocks 
enter  the  earth  is  known  as  the 
dip,  while  a  horizontal  line  at  right 
angles  to  this,  is  known  as  the 
strike  (Figs.  92  and  93).  If  we 
considered  one  side  of  the  gable 
roof  of  a  house  to  represent  an 
inclined  layer  of  rock,  the  pitch  of 


Fig.  95. 

Photograph  of  a  fold  in  the 
rocks,  Quebec,  Canada. 


210 


PHYSICAL   GEOGBAPHY. 


'fm 


Fig.  96. 
Photograph  of  a  fault  in  Arizona. 

along  a  plane  which  is  known 
result  of  the  fault- 
ing, one  side  is  left 
higher  than  the  other 
(Figs.  96  and  97). 
Sometimes  the  fault 
plane  is  nearly  ver- 
tical, and  sometimes 
nearly  horizontal  ; 
but  it  is  usually 
inclined  at  a  high 
angle.  The  amount 
of  movement  of  the 
rocks,  varies  from  a 
fraction  of  an  inch 


the  roof  would  represent 
the  dip,  and  the  ridge- 
pole, or  any  line  parallel 
to  it,  the  strike. 

In  some  cases  the 
rocks  break  or  fault,  in- 
stead of  folding  (Fig. 
96),  and  some  folds  grad- 
ually change  to  faults. 
There  is  much  complex- 
ity in  faulting,  particu- 
larly when  the  break 
extends  across  rocks 
that  have  already  been 
folded,  and  no  more  can 
be  done  here  than  to  de- 
scribe the  simplest  kind 
of  fault.  The  rocks  break 
as  the  fault  plane  ;  and  as  a 


Fig.  97. 
Photograph  of  fault  in  glacial  clay,  Massachusetts. 


THE  CRUST  OF  THE  EARTH  211 

to  several  thousand  feet.  In  the  latter  case  the  movement 
did  not  all  take  place  at  once,  but  was  the  result  of  numerous 
slippings,  perhaps  continued  for  a  long  period  of  time.  It 
is  probable  that  in  some  mountain  regions  the  rocks  are  even 
now  being  faulted ;  and  in  some  cases  the  signs  of  present 
movement  can  be  seen,  particularly  after  earthquake  shocks 
(Fig.  247). 

Volcanic  Action.  —  In  many  parts  of  the  world,  particularly 
in  some  of  the  higher  mountains,  molten  rock  and  frag- 
ments of  rock  are  reaching  the  surface  through  openings 
that  pass  down  into  the  earth,  probably  to  a  depth  of  several 
miles.  Usually  these  ejected  materials  build  a  cone  which 
we  know  as  a  volcano  (Fig.  234).  The  molten  rock  flows 
down  the  side  of  the  cone  as  a  lava  flow  and  solidifies  into 
rock.  The  fragments  are  usually  porous  like  ash,  and  in 
large  measure  this  volcanic  ash  or  pumice  also  collects  near 
the  outlet  of  the  volcano.  Some  volcanoes  send  forth  one  of 
these  and  some  the  other,  while  most  eject  now  one  and  now 
the  other. 

Some  of  the  volcanic  eruptions  are  very  violent,  while 
others  are  quite  gentle,  and  at  times  the  ash  is  sent  to 
great  distances  in  the  air.  The  lava  flows  often  extend  to 
a  distance  of  many  miles,  deluging  the  surface  over  great 
areas.  In  some  cases  the  lava  comes  to  the  surface  through 
great  cracks,  flooding  thousands  of  square  miles  of  country. 
In  earlier  geological  ages  volcanoes  existed  in  parts  of  the 
world  where  they  are  now  absent,  and  in  such  places  we 
sometimes  find  the  lava  flows  at  present  on  the  surface. 

Not  only  are  these  molten  materials  sent  to  the  surface^ 
but  they  are  found  to  be  intruded  in  many  rocks.  Since  the 
lava  comes  from  below,  it  must  pass  through  the  strata  of 
the  crust,  and  in  many  cases  it  solidifies  there  as  injections. 
The  tube,  through  which  the  lava  passes  on  its  way  to  the 


212 


PHYSICAL   GEOGRAPHY. 


crater  of  the  volcano,  becomes  filled  with  solid  lava  when 
the  volcanic  action  ceases ;  and  sometimes  it  tries  to  reach 
the  surface  along  other  planes,  breaking  the  rock  open  and 
filling  the  cracks  with  lava,  forming  dikes  (Fig.  98).  These 
are  very  abundant  in  regions  of  volcanic  action,  and  they 
often  occur  in  places  where  such  action  was  once  present, 
being  the  roots  of  old  volcanoes.     Such  dikes  are  extremely 

abundant  in  New  England, 
where  they  may  be  seen 
in  great  numbers  cutting 
across  the  rocks  of  the 
seashore. 

In  some  of  the  deep  parts 
of  the  earth,  in  the  center  of 
mountains,  these  intruded 
masses  are  of  great  size, 
sometimes  miles  in  diam- 
eter. These  great  bosses 
of  intruded  materials  are 
illustrated  by  the  granite 
areas  ;  for  these  rocks  were 
formed  in  this  way,  and 
are  now  exposed  at  the 
surface  because  the  moun- 
tain cover  has  been  worn 
away.  These  great  masses  of  molten  rock,  intruded  into 
parts  of  the  earth  at  depths  of  a  few  thousand  feet,  bring  to 
these  parts  of  the  crust  a  greater  heat  than  belongs  there, 
and  cause  many  peculiar  changes. 

Rocks  of  the  Earth's  Crust.  —  We  have  no  means  of 
knowing  the  condition  of  the  earth  at  depths  greater  than 
a  few  thousand  feet ;  but  the  rocks  at  the  very  surface  are 
quite  well  known.     There  are  three  great  groups  of  such 


Fig.  98. 

Photograph  of   a  dike   crossing  granite, 
Cape  Ann,  Mass. 


THE  CBUST  OF  THE  EARTH.  213 

rocks,  known  as  igneous,  metamorphic,  and  sedimentary. 
The  former  come  from  within  the  earth,  and  reach  their 
places  in  the  crust  as  molten  rock ;  the  second  kind  includes 
those  which  have  been  changed  or  metamorphosed,  often 
by  heat.  This  heat  has  been  derived  either  from  intruded 
volcanic  rocks,  or  from  friction  accompanying  the  folding  of 
mountains.  The  third  group  includes  those  rocks  which 
were  formed  in  water,  mostly  in  the  ocean. 

Igneous  Mocks.  —  When  the  igneous  rocks  come  from 
below  they  are  molten,  and  the  elements  of  which  they  are 
composed  are  not  definitely  united  to  form  minerals.  As 
they  cool,  the  elements  tend  to  unite  to  form  definite  com- 
pounds, which  are  minerals.  Such  rocks  are  therefore  crys- 
talline, for  they  are  composed  of  crystalline  minerals.  Since 
the  chemical  composition  of  the  lavas  varies  in  different 
places,  there  is  much  difference  in  the  rock  that  is  formed. 
Some  are  black,  like  the  trap  of  the  Palisades  of  the  Hudson, 
or  like  the  basaltic  lava  of  the  volcanoes  of  the  Sandwich 
Islands,  while  others  are  nearly  white.  The  minerals  that 
are  most  common  in  these  rocks,  are  quartz,  feldspar,  horn- 
blende, and  mica.^ 

If  a  saturated  solution  of  salt  in  hot  water  be  allowed  to 
cool  suddenly,  the  salt  forms  one  mass  of  small  crystals  ;  but 
if  several  hours  be  allowed  to  elapse  in  the  cooling,  the  crys- 
tals are  much  larger.  Just  so  in  these  igneous  rocks ;  and 
as  a  result  of  this,  some  lavas  are  of  very  fine  grain,  and 
even  glassy  (known  as  obsidian  or  natural  glass),  while 
others  are  moderately  coarse,  and  still  others  very  coarse. 
Ordinary  lava  is  fine  grained  because  it  cools  rapidly  at  the 
surface,  while  the  intruded  rocks,  such  as  granite,  are  much 

1  It  does  not  seem  profitable  to  describe  these  minerals  or  the  rocks.  If 
the  students  are  not  already  familiar  with  them,  it  would  be  well  to  have 
them  study  specimens ;  but  mere  descriptions  are  of  little  avail. 


214 


PHYSICAL   GEOGBAPHY. 


coarser,  because  they  could  not  cool  so  rapidly.  Therefore 
igneous  rocks  vary  in  two  ways,  in  coarseness  and  in  chem- 
ical composition,  and  hence  in  mineral  constitution.  All  of 
these  varieties  are  given  names,  but  their  study  belongs  to 
geology. 

Metamorphic  Rocks.  —  Though  they  were  not  molten, 
metamorphic   rocks   resemble  the  igneous  in  the  fact  that 

they  are  formed  through  the 
partial  agency  of  heat,  and  in 
the  fact  that  they  are  crystal- 
line. They  are  the  least  im- 
portant group,  but  in  some 
places,  such  as  New  England 
and  Canada,  they  are  the  most 
common  of  rocks.  They  are 
usually  banded  or  foliated,  and 
these  bands  are  often  greatly 
contorted  (Fig.  99).  Some  of 
them  are  known  to  be  the  altered  forms  of  other  rocks,  while 
the  original  condition  of  others  cannot  be  told.  We  know 
that  marble  is  the  altered  form  of  limestone,  slate  is  meta- 
morphosed from  a  clay  rock,  etc. ;  but  the  two  most  common 
metamorphic  rocks  —  gneiss  and  schist  —  cannot  usually  be 
traced  to  their  original  condition.  They  are  generally  very 
hard  rocks. 

Sedimentary  Rocks.  —  The  most  important  of  the  groups 
is  that  of  the  sedimentary  rocks,  which  are  mostly  sedi- 
ments formed  in  the  ocean.  They  may  be  divided  into  three 
classes, — mechanical,  chemical,  and  organic.  The  organic 
rocks  are  formed  from  the  remains  of  animals  or  plants,  the 
coal  illustrating  the  latter  and  limestone  the  former.  The 
great  ocean  deposit  of  Globigerina  ooze  (page  164),  and 
the  coral  reefs,  are  organic  sediments.     Chemical  sediments 


Fig.  99. 
Contorted  limestone. 


THE  CBUST  OF  THE  EARTH. 


215 


are  not  of  sufficient  importance  to  occupy  space  here,  but 
the  most  important  group  is  the  mechanical. 

The  rocks  of  the  earth's  surface  are  being  destroyed  by 
various  means,  and  the  fragments  are  being  transported 
toward  the  sea.  Since  some  of  the  minerals  cannot  with- 
stand the  action  of  the  weather,  the  rocks  actually  decay 
and  form  fragments ;  and  as  they  change  and  crumble,  the 
rock  falls  to  pieces,  thus  making  the  beginning  of  a  soil. 
Every  rain  takes  some  of  these  pulverized  rock  particles  and 
carries  them  to  a  stream,  where  they  begin  their  journey 
to  the  sea  (Fig.  122).  To  these  are  added  others  which 
the  stream  takes 
from  its  bed;  and 
in  the  ocean  there 
are  added  those 
that  the  waves 
rasp  from  the 
land.  In  the 
ocean  these  ac- 
cumulate in  lay- 
ers, the  coarsest 
where  the  waters 
are  in  most  rapid 
motion,  and  the 
finest  where  they 
are  so  still  that 
the  particles  may  settle.  The  coarser  rocks  with  pebbles, 
such  as  those  of  the  beaches,  are  known  as  conglomerates ; 
the  very  finest  produce  clay  rocks,  such  as  the  shales ;  and 
the  intermediate  sandstones  are  composed  of  sandy  grains 
of  the  very  durable  mineral  quartz. 

Deposition  of  Sedimentary  Rocks.  —  Reaching  the  ocean, 
these  rock  fragments  are  strewn  over  the  bottom  of  the  sea, 


Fig.  100. 
Stratified  shale  rocks  in  a  gorge  near  Ithaca,  N.Y. 


216 


PHYSICAL   GEOGBAPHY. 


Shale 


W0§B  Sandstone 


Shale 
Conglomerate 


p£--^E-=:  Shale 


particularly  near  the  coast,  because  here  the  ocean  waters  are 
so  quiet  that  the  particles  must  settle.  In  quiet  bays,  very 
fine-grained  rocks  may  be  deposited  close  to  the  shore  ;  but 
on  more  exposed  coasts,  the  sediments  of  the  shore  line  are 

coarse-grained,  and  as  the  distance  from 
^^^(^^<^  the    coast   increases,  they   become   finer 

O  7  7  •'  */ 

Sandstone  ^^  texture.     Since  the  ocean   bottom  is 

Shale  usually  nearly  level,  these  fragments  are 

^Conglomerate    spread  out  in  layers  which   are   nearly 

horizontal,  though  where  the  bottom  is 
inclined,  the  layers  are  inclined  with  it. 
Sometimes  the  supply  of  sediment  varies, 
either  in  amount  or  in  kind,  and  so  one 
layer  may  be  deposited  on  another ;  and 
this  gives  the  stratification  that  is  so 
characteristic  of  most  sedimentary  rocks 
(Fig.  100).  We  may  have  a  layer  or 
stratum  of  sand  resting  on  one  of  clay, 
and  upon  this  a  layer  of  limestone,  etc. 
(Fig.  101). 

Sedimentary  rocks  are  now  being 
formed  over  the  entire  floor  of  the  ocean ; 
but  at  a  greater  distance  than  a  few  score 
of  miles  from  the  land,  the  sediments  for 
the  most  part  are  organic.  The  greater 
part  of  the  rocks  of  the  land  are  sedi- 
mentary in  origin ;  and  most  of  them  furnish  evidence  that 
they  were  formed  in  the  ocean  near  the  shore.  This  proves 
that  they  must  have  been  elevated  from  the  sea ;  and  we 
know  full  well  that  the  continents  are  largely  built  of 
materials  that  were  formed  in  the  ocean  not  far  from  the 
shore.  Sometimes  these  rocks  have  a  thickness  of  thou- 
sands of  feet,  and  yet  they  are  made  up  of  sediments  that 


:^  >  '  ^ 


C^SS 


3_^^J, 


Shaly 
Sandstone 


Shale 
Sandstone 

Limestone 


Fig.  101. 

Section   showing   alter 
nation  of  strata. 


THE  CBUST  OF  THE  EARTH. 


217 


Fig.  102. 
An  unconformity  in  horizontal  rocks. 


were  laid  down  in  the  shallow  waters  near  the  coast.  The 
only  way  in  which  this  could  happen  is  by  a  continued 
sinking  of  the  bottom.  Therefore  the  sedimentary  rocks 
teach  us  that  parts  of  the  sea  bottom  continued  to  sink  for  a 
long  time,  and  were  then  elevated  to  form  continents. 

Other  movements  of  the 
crust  are  also  shown  by 
some  of  these  rocks.  At 
times  there  are  unconform- 
ities (Figs.  102  and  103) : 
that  is,  rocks  made  in  the 
sea,  rest  on  other  sea- 
formed  strata  which  were  deposited  at  an  earlier  period, 
and  have  since  been  land.  Thus  we  have  in  these  cases, 
(1)  deposit  in  the  ocean,  (2)  elevation  to  land,  (3)  depression 
beneath  the  sea,  and  (4)  a  second  elevation.  In  some  cases 
there  are  numerous  such  unconformities,  showing  successive 
changes.     These,  and  other  facts,  prove  that  the  crust  of  the 

earth  is  almost  con- 
stantly in  movement. 
Consolidation  of 
Sedimentary  Rocks. 
—  The  rocks  of  the 
sea  are  soft  and  un- 
consolidated, while 
those  of  the  land  are 
generally  hard  and 
compact.  The  consolidation  of  rocks  is  a  simple  process, 
generally  resulting  from  heat,  pressure,  the  deposition  of 
some  cement,  or  a  combination  of  several  such  causes.  In  a 
hydraulic  press  we  can  consolidate  clay;  and  in  a  similar 
way,  the  great  weight  of  the  strata  of  the  crust,  furnishes 
the  necessary  pressure  for  the  natural  consolidation  of  rocks. 


Fig.  103. 

An  unconformity  in  inclined  rocks, 
land  surface. 


A,  B,  old 


218  PHYSICAL   GEOGRAPHY. 

Bricks  are  consolidated  by  heat,  and  in  the  earth  heat  often 
acts  in  a  similar  manner.  All  rocks  in  the  earth  are  filled 
with  water  which  is  slowly  percolating  through  them.  This 
water  is  dissolving  substances  from  one  place  and  depositing 
them  in  others,  and  in  this  way  many  rocks  are  being  con- 
solidated. Carbonate  of  lime  and  some  compound  of  iron, 
are  the  common  rock  cements ;  and  these,  perhaps  aided  by 
one  of  the  other  causes,  bind  the  rock  particles  together. 

Geological  Chronology.  —  By  a  study  of  the  rocks,  the  main 
facts  of  geological  history  have  been  determined  in  a  more 
or  less  satisfactory  manner.  We  know  something  of  the 
history  of  the  globe,  and  the  rocks  form  the  pages  and  chap- 
ters of  this  history.  The  rock  record  is  often  very  imper- 
fect. Some  pages,  and  at  times  entire  chapters,  are  missing  ; 
but  enough  still  remains  to  furnish  a  basis  of  value.  One 
thing  shown,  is  that  the  world  is  very  old,  and  that  no 
statement  of  the  history  in  years  or  centuries  is  possible. 
Therefore  there  is  no  chronology  of  the  kind  that  we  are 
accustomed  to  use  in  recording  human  events. 

In  many  of  the  sedimentary  rocks  there  are  fossils,  which 
are  the  entombed  remnants  of  animals  and  plants  that  lived 
when  these  rocks  were  formed  (Fig.  104).  If  1000  feet 
of  rocks  are  found,  one  laid  down  upon  the  other,  and  if 
these  contain  fossils,  there  is  preserved  a  record  of  some  of 
the  organisms  that  lived  while  these  rocks  were  being  depos- 
ited. By  a  very  careful  study  of  the  fossils  of  various  parts 
of  the  earth,  a  nearly  continuous  record  of  the  life  of  the 
globe  has  been  obtained,  from  near  the  beginning  of  life  to 
the  present  time.  It  is  found  that  in  the  lowest  rocks  —  that 
is,  in  the  oldest  —  the  animal  remains  are  only  of  low  types. 
At  first  there  were  no  land  animals  and  plants  ;  and  in  the 
sea,  the  only  animals  were  of  types  lower  than  the  true 
fishes.     The  fishes  appeared,  then  reptiles,  birds,  and  mam- 


THE  CRUST  OF  THE  EARTH. 


219 


mals  in  succession ;   and  this  evolution  from  lower  to  higher 
forms  is  noticed  even  among  the  subdivisions  of  life. 

Therefore,  upon  examining  the  fossils  from  a  rock,  a  geol- 
ogist can  tell  in  what  part  of  the  earth's  history  they  lived, 
and  to  what  stage  in  this  history  the  rock  belongs.  It  is 
like  the  study  of  prehistoric  man,  which  is  based  on  the 
implements  he  used.     Certain   kinds  of   stone   implements 


Fig.  104. 
Photograph  of  a  rock  containing  fossils. 

mark  the  paloeolithic  age,  others  the  neolithic ;  bronze 
implements  mark  a  higher  stage,  etc.  This  does  not  mean 
an  age  in  any  sense  in  which  years  are  used,  but  rather 
a  stage.  One  of  these  stages  may  represent  a  thousand 
years,  another  several  thousand ;  but  each  one  represents  a 
stage  different  from  that  which  preceded  and  succeeded. 

So  it  is  with  the  geological  chronology.     We  have  abso- 
lutely no  basis  for  division  into  periods  of  years  ;  but  we  can 


220 


PHYSICAL   GEOGBAPHT. 


divide  the  history  into  stages,  each  stage  representing  some 
advance  in  the  development  of  life  on  the  globe.  -For  this 
purpose,  names  are  used  to  signify  the  stages,  as  is  indicated 
in  the  table  below,  which  is  a  simple  one  from  which  the 


TABLE    OF    GEOLOGICAL    AGES. 


CENOZOIC 
TIME. 

Age   of 

mammals. 

Quaternary. 

Man  assumes  importance,  particularly  in 
the  upper  part.  In  the  first  half  the  Gla- 
cial Period  prevailed. 

Tertiary. 

Mammals  develop  in  remarkable  variety, 
and  to  great  size,  while  reptiles  diminish. 

MESOZOIC 
TIME. 

Age  of  reptiles. 

Cretaceous. 

Birds  begin  to  become  important,  reptiles 
continue,  and  higher  mammals  begin. 
Land  plants  and  insects  of  high  types. 

Jurassic. 

Keptiles  and  amphibia  continue  to  be 
predominant. 

Triassic. 

Amphibia  and  reptiles  develop  remark- 
ably.     Mammals  of  lovs^  forms  appear. 

PALEOZOIC 
TIME. 

The  age  of 
invertebrates. 

Carboniferous. 

Land  plants  assume  great  importance. 

Devonian. 

Fishes  begin  to  be  abundant. 

Silurian. 

Invertebrates  prevail.  ^ 

Cambrian. 

No  forms  higher  than  invertebrates. 

In  part 
AZOIC  TIME. 

No  fossils  known 

Archean. 

Mostly  metamorphic  rocks,  perhaps  in 
part  the  original  crust  of  the  earth. 

1  Invertebrates  of  course  continue  dow^n  to  the  very  present ;  but  until  the 
Devonian,  they  were  the  most  important  group.  The  same  is  true  of  fishes, 
which  begin  to  be  abundant  in  the  Devonian,  but  continue  down  to  the  present. 


THE  CRUST  OF  THE  EARTH.  221 

subdivisions  are  omitted.  Each  of  these  ages  represents  the 
lapse  of  immense  periods  of  time,  perhaps  hundreds  of 
thousands  of  years  ;  but  no  interpretation  of  years  is  to  be 
placed  upon  them,  nor  should  it  be  assumed  that  they  are  of 
equal  length.  The  Carboniferous  represents  the  stage  in 
the  earth's  history  when  plants  had  reached  a  certain  type 
of  development  upon  the  land,  etc. 

Age  of  the  Earth.  —  As  has  been  said,  we  have  no  basis  for 
an  estimate  of  the  age  of  the  earth.  By  some  scientists  esti- 
mates have  been  made  upon  one  basis  or  another,  and  these 
have  ranged  between  3,000,000  and  2,400,000,000  years, 
though  the  majority  have  estimated  a  few  hundred  million 
years.  Since  these  estimates  were  made  by  very  different 
men,  upon  entirely  different  facts,  they  have  the  one  great 
value  that  they  prove  the  great  age  of  the  earth. 

One  cannot  go  far  in  the  study  of  geology  without  being 
convinced  by  the  overwhelming  evidence  that  the  earth  is 
exceedingly  old.  To  attempt  to  explain  the  phenomena  of 
the  earth's  surface  upon  the  basis  of  single  years  or  centu- 
ries, would  be  as  fruitless  as  would  be  the  attempt  of  the 
astronomer  to  explain  the  facts  of  the  solar  system  on  the 
supposition  that  the  planets  were  at  distances  of  a  few 
thousand  miles.  The  only  way  to  have  the  force  of  this 
statement  impressed  in  all  of  its  fulness,  is  to  study  the  earth 
with  the  eyes  of  a  geologist ;  and  in  a  study  of  this  nature 
only  the  beginning  of  this  can  be  attempted.  Still  it  is 
necessary  that  this  fact  should  be  accepted  at  the  outset. 
Just  as  the  student  of  astronomy  gazes  at  the  stars,  and, 
upon  faith  alone,  accepts  the  statement  that  these  bodies  lie 
millions  and  even  billions  of  miles  from  him,  so  the  student 
of  geology  or  physical  geography  must  commence  the  study 
of  the  earth  with  the  belief  that  the  history  which  it  has 
passed  through  has  occupied  not  years,  nor  thousands,  nor 


222  PHYSICAL    GEOGRAPHY. 

even  hundreds  of  thousands,  but  millions  and  probably  hun- 
dreds of  millions  of  years.  The  evidence  is  overwhelming, 
and  no  geologist  finds  reason  to  doubt  it. 

The  gorge  of  Niagara,  200  or  300  feet  deep,  and  7  miles 
long,  has  taken  not  far  from  10,000  years  for  its  formation ; 
how  much  longer  was  the  time  occupied  in  forming  the  caiion 
of  the  Colorado,  whose  length  is  300  miles,  and  whose  depth 
in  places  is  over  a  mile !  Yet  these  were  formed  in  late 
stages  in  the  development  of  the  continent. 

We  watch  a  volcano  for  a  century,  and,  at  the  end  of  that 
time,  find  its  general  form  to  be  the  same  as  at  the  begin- 
ning ;  yet  most  of  the  volcanic  cones  of  the  world  were 
begun  not  earlier  than  the  commencement  of  the  Tertiary. 
Studying  the  rate  of  deposit  of  the  sedimentary  rocks  of  the 
ocean,  we  find  that,  even  when  the  deposit  is  rapidly  made, 
but  a  few  feet  are  laid  down  in  a  single  century ;  yet,  in 
some  places,  many  thousand  feet  of  rocks  have  thus  been 
deposited,  one  layer  upon  another.  In  the  Appalachian 
Mountains  there  are  fully  40,000  feet  of  these  strata,  and 
they  were  all  formed  in  the  Palaeozoic.  How  many  scores 
of  centuries  do  these  represent ! 

This,  and  other  evidence  equally  striking,  is  what  has 
driven  the  geologists  to  the  conclusion  (for  a  long  time 
opposed,  as  was  the  present  astronomy  when  first  proposed) 
that  the  age  of  the  earth  is  incalculable,  but  great, — a  conclu- 
sion now  quite  universally  accepted.  It  is  the  basal  concep- 
tion of  geology,  and  must  be  accepted  at  the  beginning.  To 
it  must  be  added  the  conception  of  the  fact  that  the  earth  is 
changing.  These  changes,  so  slow  as  to  be  almost  impercep- 
tible in  a  single  lifetime,  when  allowed  long  periods  of  time 
for  their  action,  will  produce  the  most  profound  and  stupen- 
dous revolutions.  From  this  time  on  we  will  study  the  crust 
of  the  earth  as  a  thing  of  constant  change,  and  of  great,  but 


THE  CRUST  OF  THE  EABTH.  223 

indefinite  age.  The  present  is  but  one  stage  in  its  history : 
there  has  been  a  past,  and  there  will  be  a  future,  just  as  is 
the  case  with  the  history  of  man  himself. 


REFERENCE   BOOKS. 

LARGER    BOOKS    OP    REFERENCE. 

Geikie. — Text  Book  of  Geology.  Macmillan  &  Co.,  New  York.  Third 
edition  (revised),  1893.  8vo.  $7.50.  (The  most  complete  English  text 
book.) 

Dana.  —  Manual  of  Geology.  American  Book  Co.,  New  York.  Fourth 
edition  (revised),  1895.  8vo.  $5.00.  (The  standard  American  reference 
book  ;  thoroughly  revised  to  date. ) 

Le  Conte. — Elements  of  Geology,  American  Book  Co., New  York.  Revised 
edition,  1891.     8vo.     $4.00.     (A  very  valuable  book  of  reference. ) 

smaller  text  books. 

Geikie.  —  Class  Book  of  Geology.  Macmillan  &  Co.,  New  York.  Third 
edition,  1892.     12mo.     $1.10. 

Jukes-Browne.  —  Handbook  of  Physical  Geology.  Macmillan  &  Co., 
New  York.     Second  edition,  1892.     12mo.     $1.75. 

Le  Conte.  —  Compend  of  Geology.  American  Book  Co.,  New  York,  1894. 
12mo.     $1.20. 

Dana.  —  Text  Book  of  Geology.  American  Book  Co.,  New  York.  Fourth 
edition,  1884.     8vo.     $2.00. 

Winchell.  —  Geological  Studies.  Griggs,  Chicago.  Fourth  edition,  1892. 
12mo.    $2.50. 

This  list  contains  only  a  few  of  the  many  excellent  text  books  of  geologj' ; 
and  others  are  referred  to  at  the  end  of  the  next  chapter. 

In  some  of  the  states  of  the  Union,  there  are  geological  surveys  which  have 
published  reports  in  which  one  may  often  find  a  description  of  his  own 
region.  Among  others,  the  following  states  have  recently  had  such  surveys : 
New  York,  New  Jersey,  Pennsylvania,  North  Carolina,  Georgia,  Alabama, 
Mississippi,  Texas,  Arkansas,  Ohio,  Michigan,  Minnesota,  Missouri,  Kansas, 
Iowa,  South  Dakota,  and  California.  Where  the  reports  cannot  be  obtained 
from  the  state  geologist,  they  can  often  be  found  in  second-hand  stores. 


CHAPTER   XIII. 

DENUDATION   OF   THE  LAND. 

Underground  Water.  —  When  rocks  are  deposited  in  the 
ocean,  the  crevices  between  the  particles  of  sediment  are 
filled  with  water.  In  even  the  densest  of  rocks  there  are 
cavities,  and  through  all  of  these,  water  is  slowly  percolating 
as  underground  water.  Added  to  the  supply  originally  in 
the  rocks,  there  is  a  constant  body  of  water  entering  at  the 
surface.  When  rain  falls  upon  the  land,  a  part  is  returned 
to  the  air  by  evaporation,  a  second  portion  flows  away  as 
surface  water,  and  a  third  part  sinks  into  the  ground.  This 
last  portion  commences  an  underground  journey  through  the 
strata,  in  the  course  of  which  much  work  is  done.  It  moves 
along  the  larger  crevices,  and  also  slowly  passes  through  the 
very  rock  itself.  That  this  water  is  actually  present  in  the 
strata,  is  shown  by  the  fact  that  wells  may  be  constructed  in 
them ;  and  even  in  the  deepest  mines,  water  is  found  to  be 
present  in  the  rocks. 

Some  minerals  are  soluble  in  water,  and  the  hardness  of 
certain  waters  is  due  to  the  fact  that  they  contain  mineral 
matter  in  solution.  All  underground  water  is  engaged  in 
this  work  of  dissolving  rock  materials.  While  pure  water 
has  but  little  power  of  solution  for  ordinary  minerals,  when 
it  is  supplied  with  certain  impurities  its  solvent  power  is 
greatly  increased.  There  are  many  substances  which  add 
poAver  to  this  percolating  water,  but  those  which  are  most 
commonly  present,  are  the  various  acids  supplied  by  decaying 

224 


DENUDATIOJ^  OF  THE  LAND. 


225 


vegetation.  The  humous  and  humic  acids  and  carbonic  acid 
gas  are  most  commonly  present  in  underground  water ;  and 
armed  with  these,  it  possesses  great  solvent  power.  When 
the  water  has  percolated  to  a  considerable  depth  in  the  earth, 
its  temperature  is  so  raised  that  its  power  is  greatly  increased. 
In  some  cases  it  obtains  a  temperature  higher  than  the  boil- 
ing point  at  the  surface ;  and  then  it  becomes  a  powerful 
solvent,   partic- 


1 

1 

^^ 

i 

'"'  i  / 

i 

/  __    i 

ularly  if  it  is 
armed  with 
acids  or  alkalies. 
When  it  reaches 
the  surface  in 
the  form  of  a 
spring,  we  very 
often  find  proof 
that  under- 
ground  water  is 
engaged  in  this 
work  of  solu- 
tion. Many  of 
these  are  min- 
eral springs,  and 
at  times,  deposits 
of  iron,  or  other 
substances,  are 
made  where  the  water  reaches  the  surface.  When  hot  water 
escapes  at  the  surface,  as  is  the  case  in  the  geyser  region 
of  the  Yellowstone  Park,  extensive  chemical  deposits  of  rock 
are  sometimes  formed  around  the  springs  (Fig.  105).  The 
reason  for  the  deposit  of  these  substances,  is  sometimes  that 
the  temperature  of  the  water  is  lowered,  and  its  solvent 
power   thereby  decreased;   in  other   cases  it  is  due  to  the 

Q 


Fig.  lUo. 

Deposits  of  carbonate  of  lime,  Pulpit  terrace,  Mammoth 
Hot  Springs  of  Yellowstone  Park. 


226  PHYSICAL    GEOGRAPHY, 

escape  of  certain  gases  which  gave  to  it  much  of  its  power ; 
and  it  is  often  the  result  of  chemical  changes  in  the  presence 
of  the  air.  Even  in  the  earth,  for  one  reason  or  another,  the 
water  at  times  deposits  some  of  its  dissolved  load.  This  is 
one  of  the  ways  in  which  rocks  are  cemented  ;  and  it  appears 
to  be  one  of  the  causes  for  the  formation  of  some  of  the 
valuable  mineral  deposits. 

Underground  water  is  also  engaged  in  the  work  of  chang- 
ing some  of  the  minerals  of  the  rocks.  It  actually  causes 
a  decay  of  some  minerals,  and  brings  about  very  important 
changes  in  others.  This  is  one  of  the  ways  in  which  the 
rocks  are  broken  into  fragments,  and  soils  formed.      This 


Fig.  106. 
Diagram  to  illustrate  the  formation  of  caverns. 

work  of  underground  water  is  not  confined  to  the  surface 
layers,  but  extends  to  considerable  depths  in  the  earth. 
However,  from  our  present  standpoint,  the  most  important 
changes  are  those  which  are  produced  nearly  at  the  surface, 
and  these  are  again  referred  to  below. 

The  Formation  of  Caverns.  —  Limestone  is  one  of  the  most 
soluble  of  the  rocks ;  and  in  many  of  the  regions  where  this 
exists,  the  solvent  action  of  underground  water  goes  so  far 
as  to  actually  dissolve  cavities  in  the  strata  (Fig.  106).  It 
sinks  into  the  ground  through  depressions,  or  sink  holes 
(Fig.  107),  and  passes  along  planes  of  weakness,  which  it 
enlarges  by  solution ;  and  in  some  cases,  this  underground 
water  assumes  the  form  of  true  subterranean  rivers,  which 


DENUDATION   OF  THE  LAND. 


227 


Fig.  107.    A  sink  hole  in  a  limestone  region. 


are     sometimes 

several  miles  in 

length.  The  cav- 

erns  (Fig.  106) 

thus  formed,  are 

very  irregular ; 

and  some,  such 

as  the  Mammoth 

Cave    of    Ken- 
tucky, and  Lu- 

ray  Cave,  have 

been      explored 

and    opened   to 

tourists ;   but  there  are  thousands  which  have  never  been 

entered. 

In  some  of  these 
caves,  the  water  that 
percolates  through  the 
roof,  deposits  columns 
and  pendants  of  car- 
bonate of  lime,  which 
often  produce  most 
beautiful  effects. 
When  these  reach 
from  the  roof  they  are 
known  as  stalactites 
(Fig.  108) ;  and  when 
they  extend  from  the 
floor,  they  are  called 
stalagmites;  while  by 
the  junction  of  these, 
columns      are       often 

Fig.  108.    Stalactites  in  cavern  of  Luray.  formed    from    floor   tO 


228 


PHYSICAL   GEOGRAPHY. 


roof.  They  are  formed  because  on  entering  tlie  cave  the  water 
loses  some  of  the  carbonic  acid  gas  which  gave  to  it  its  sol- 
vent powers,  and  thereby  has  its 
ability  to  hold  in  solution  de- 
creased. By  the  gradual  lower- 
ing of  the  land,  the  roofs  of 
these  caverns  are  sometimes  de- 
stroyed, and  the  streams  that 
occupy  them  are  changed  to  sur- 
face rivers.  Where  a  part  of  the 
roof  remains,  a  natural  bridge  is 
sometimes  formed  (Figs.  106  and 
109). 

Springs  and  Artesian  Wells.  — 
Underground  water  often  finds 
channels  of  escape  to  the  surface  ; 
and  where  it  reaches  the  surface, 
springs  are  produced.  This  escape 
may  be  along  fault  planes,  or  other 
breaks  in  the  rocks  (Fig.  110),  or  it  may  be  at  the  outlet 
of  a  subterranean  stream  which  passes  through  a  cavern 
(Fig.   131);    but  the   majority   of   springs   occur   where    a 


iftft^^SI 

wNb 

1                 0     ■  .1      .js^f'*'-^- 

%t^ 

,  i^^     ■^t<^-MJ'~    »■ 

V^/^ITj.. 

■--^ 

f  ^^i^Mi^^ 

...    *** 

Wi^.  IM^B 

^PIP 

r5f              ^,K,  • 

f^ 

ittLiiar^'^        -^ 

:,       ^ 

^m 

wumL.^ .  ..-.^y^aL.s 

Fig.  109. 
The  Natural  Bridge,  Virginia. 


Fig.  110. 
A  spring  formed  along  a  fault  plane  (/,  s) ;  (a,  a)  impervious  layers ;  (6)  porous 
stratum.  Arrows  show  the  course  followed  by  the  underground  water  leading 
to  the  spring  (s) . 


DENUDATION   OF  THE  LAND. 


229 


loose-textured   rock  rests  upon   a   less  permeable   one,  and 
where  this  junction  is  exposed  at  the  surface   (Fig.  111). 


Fig.  111. 
Hillside  spring  {s)  at  junction  of  permeable  layer  (a)  and  impervious  layer  (6). 

This  is  particularly  liable  to  happen  on  hillsides  where  a 
layer  of  sand  rests  upon  a  stratum  of  clay. 

In  the  earth,  certain  strata  are  more  permeable  to  water 
than  are  others;  and  under  some 
circumstances  the  conditions  fa- 
voring the  production  of  arte- 
sian wells  (Fig.  112)  may  be 
present.  Sandstones  are  the 
most  permeable  of  rocks,  and 
when  a  sandy  layer  crops  out  at 
the    surface,   the   water   readily 


soaks  into  it.     If  such  a  layer  is 


1 


covered  and  underlaid  by  a  more 
dense  rock,  such  as  a  clay  stra- 
tum, the  water  that  enters  the 
sandy  layer  is  in  large  measure 
imprisoned  within  it.  If  under 
such  conditions  the  strata  dip 
into  the  earth,  the  water  in  the 
sandstone  passes  down  this  layer 
between  the  two  enclosing  walls. 
As  a  result  of  the  weight  of  the 
column  of  water  in  the  stratum,  it  is  under  a  considerable 


Fig.  112. 
Artesian  well. 


230 


PHYSICAL   GEOGBAPHY. 


pressure ;  and  this  is  sufficient  to  force  it  upward  toAA^ard 
the  surface,  to  a  height  nearly  as  great  as  that  of  the  place 
where  the  water  enters  the  ground. 

If  this  Avater-bearing  lajer  is  pierced  by  a  well-boring, 
the  water  AA'ill  rise  in  the  aacII  as  high  as  the  pressure 
can  force  it ;  and  if   the  place  at  which  the  well  is  bored, 


Fig.  113. 

Conditions  favoring  artesian  wells  (c,  c,  c),  where  the  rocks  are  inclined  in  a 
single  direction.    Porous  sandy  layer  (o),  impervious  strata  (&,  6). 

has  a  lower  elevation  than  the  AA^ater  head,  the  water  from 
the  stratum  may  reach  the  surface  as  a  fountain,  forming 
an  artesian  w^ell  (Fig.  113).  When  this  condition  is  en- 
countered in  a  syncline,  there  are  two  water  heads,  and  this 
greatly  favors  the  formation  of  an  artesian  aa'cII  (Fig.  114). 


Fig.  114. 

Artesian  wells  {c,c,c),  where  rocks  are  folded  into  the  form  of  a  syncline; 
(a,  a)  porous  layer  between  two  impervious  layers  (6,  6). 


In  eastern  Texas,  there  is  a  water-bearing  stratum  extending 
over  a  great  area  (Fig.  113),  Avhich  has  been  tapped  at  nu- 
merous places,  and  which  furnishes  abundant  Avater  supply 
for  several  cities ;  and  the  same  is  true  of  South  Dakota 
and  elscAA'here.  In  many  parts  of  the  Avest,  artesian  wells 
are  A^ery  useful  for  purposes  of  irrigation.     It  often  happens 


DENUDATION  OF  THE  LAND. 


231 


that  the  water  does  not  rise  quite  to  the  surface,  and  then 
pumps  are  necessary,  the  pumping  often  being  done  by  wind- 
mills. 

Durability  of  Rocks.  —  There  is  a  great  difference  in  the 
ability  of  rocks  to  withstand  the  action  of  the  agents  which 
are  tending  to  destroy  them.  Some,  such  as  granites,  are 
very  hard ;  others,  such  as  limestones  and  shales,  are  soft. 
Many  rocks  that  are  hai'd 
are  chemically  weak,  and 
their  minerals  are  easily 
dissolved,  or  are  readily 
altered.  By  these  proc- 
esses, such  strata  are 
caused  to  decay  and  crum- 
ble. Some  rocks  are  loose 
in  texture  and  readily  en- 
tered by  percolating  water, 
while  others  are  dense  and 
quite  impermeable.  Otlier 
things  being  equal,  tlie 
latter  are  less  easily  de- 
stroyed than  those  that  are 
loose  in  texture.  Some 
which  are  mechanically 
hard  are  readily  destroyed 
by  chemical  means.  In 
the   later   pages,    when   a 

hard  rock  is  mentioned,  the  term  is  used  not  merely  in  the 
mechanical  sense,  but  as  a  synonym  of  resistant.^  All  rocks, 
no  matter  how  resistant  they  may  be,  are  capable  of  being 

1  That  is  to  say,  a  hard  or  resistant  rock  is  one  which  withstands  all 
attacks,  whether  mechanical  or  chemical,  more  successfully  than  less  durable 
rocks,  as  explained  in  the  next  section. 


Fig.  115. 

Rock  pillars,  Garden  of  Gods,  Colorado. 
Soft  rock  capped  by  a  harder  one  and 
hence  protected  from  destruction. 


232 


PHYSICAL   GEOGRAPHY, 


Plate  20. 

Earth  columns,  New  Mexico.    Illustrating  the  greater  resistance  of  the  thin,  hard 
layers  in  soft  clay.    The  beginning  of  the  formation  of  rock  pillars. 


DENUDATION  OF  THE  LAND. 


233 


destroyed ;  but  there  is  a  difference  in  their  power  of  resist- 
ing destruction  (Fig.  115  and  Plate  20). 

Weathering.  —  When  exposed  to  the  air,  or  to  the  weather, 
rocks  are  destroyed  by  various  agents  which  may  be  included 
under  the  general  heading  of  weathering.  These  agents  are 
both  chemical  and  mechanical.  Already  some  of  the  chemical 
changes  have  been  noted  in  the  section  on  underground  water. 
Soluble  minerals  are  taken  from  the  rocks,  and  those  that  are 
left  are  then  less  firmly  bound  together.     The  same  result  is 


Fig.  116. 
The  crumbling  of  granite  by  disintegration  of  the  minerals. 


brought  about  by  the  change  of  minerals  during  the  passage 
of  water  through  them.  Usually  the  change  leaves  the  rock 
less  firm  than  it  was  at  first,  and  it  often  produces  a  clayey 
product  in  the  place  of  the  firm  mineral  that  was  originally 
present.  These  chemical  changes  are  particularly  liable  to 
happen  in  the  crystalline  rocks,  which  were  formed  by  the 
aid  of  heat  (Fig.  116).  When  exposed  to  the  air  and  water, 
the  minerals  that  cooled  from  a  molten  condition  are  found 
to  be  unstable  and  liable  to  change.  Some  minerals,  such  as 
quartz,  resist  this  destruction,  and  this  is  why  we  have  fresh 


234  PHYSICAL   GEOGRAPHY. 

quartz  grains  in  sandstones  that  have  been  produced  by  the 
decay  of  rocks  in  which  quartz  was  one  constituent.  The 
clay  of  such  rocks  as  shale  is  mostly  the  product  of  this 
rock  decay.  Another  result  of  these  changes  is  to  furnish 
dissolved  mineral  substances  to  river  water,  and  hence  to 
the  sea. 

Of  the  mechanical  agents,  perhaps  the  most  important  is 
that  of  change  in  temperature,  which,  however,  affects  only 
the  very  surface  rocks.  In  the  regions  which  experience 
great  temperature  ranges,  the  rocks  become  warmed  during 
the  day  and  cooled  at  night.  This  introduces  an  alternate 
expansion  and  contraction,  which  causes  fragments  to  be 
split  from  the  rock  surface.  If  the  temperature  descends 
below  the  freezing  point,  as  is  the  case  in  the  high  temperate 
and  arctic  latitudes,  the  water  in  the  rock  crevices  is  frozen, 
and,  by  the  consequent  expansion,  fragments  are  pried  off. 
This  is  a  very  important  action  on  mountain  tops  (Fig.  224) 
and  on  exposed  ledges  in  cold  countries.  A  snow  cover- 
ing tends  to  check  this  action.  Naturally,  those  rocks  with 
porous  texture  are  more  open  to  the  attacks  of  frost  than 
those  which  are  compact ;  and  open-textured  rocks  are  also 
more  liable  to  be  readily  destroyed  by  percolating  water 
than  are  those  of  fine  and  compact  grain. 

Plants  are  also  important  agents  of  weathering,  and  their 
action  is  both  chemical  and  mechanical.  They  act  chemi- 
cally by  furnishing  to  percolating  water  many  of  the  sub- 
stances with  which  it  is  able  to  dissolve  and  alter  the 
minerals  ;  and  they  also  extract  mineral  matter  from  the 
soil  in  water  absorbed  through  the  roots.  The  mechani- 
cal action  of  plants  is  mainly  that  of  their  roots.  These 
enter  the  rock  crevices,  and  upon  growing,  enlarge  these 
cavities,  causing  the  rocks  to  crumble  (Fig.  117).  This 
action  may  often  be  seen  upon  a  ledge  on  which  lichens  are 


DENUDATION   OF  THE  LAND.  235 

growing ;  and  the  roots  of  trees  are  doing  a  very  important 
work  of  this  nature,  because  they  extend  through  the  soil 
to  the  rock  beneath. 

Even  animaU  are  aiding  in  this  work,  particularly  those 
that  burrow  in  the  earth.  Earthworms  are  of  great  impor- 
tance in  this  respect,  for  they  are  engaged  in  the  constant 
work  of  pulverizing  the  soil.  The  action  of  the  agents 
above  described,  is  not  confined  to  the  solid  rock,  but  it  is 


Fig.  117. 
Roots  of  a  tree  breaking  a  rock  into  fragments. 

constantly   in   progress   in   the    soil,    the   tendency   always 
being  to  make  this  finer  in  texture. 

The  results  of  this  action  of  weathering  are  most  wide- 
spread. All  over  the  land,  in  nearly  every  place,  the  rocks 
are  being  destroyed  by  these  agents ;  and  weathering  is  the 
most  important  single  cause  for  the  destruction  of  the  strata 
and  the  melting  down  of  the  surface  of  the  land.  Weather- 
ing is  more  rapid  in  some  places  than  in  others.  On  the 
cold  mountain  tops,  its  action  is  rapid  (Fig.  224),  as  it  is  also 
in  regions  of  moisture.  On  the  other  hand,  in  arid  regions 
where  rain  is  uncommon,  weathering  is  relatively  slow,  as 


1'! 


236 


PHYSICAL   GEOGRAPHY. 


it  is  also  in  regions  where  a  deep  soil  covering  protects  the 
rocks.  Upon  exposed  ledges,  weathering  is  rapid  ;  and  this 
is  particularly  true  of  cliffs,  where  the  fragments  drop  to 
the  base  in  the  form  of  a  talus  (Figs.  118,  122),  leaving  the 
rock-face  bare  to  future  attacks.  Then  also,  weathering  is 
more  rapid  in  some  kinds  of  rocks  than  in  others. 


Fig.  118. 
Talus,  valley  of  Rio  Grande,  New  Mexico. 


The  great  result  of  weathering  is  the  lowering  of  the  land 
surface ;  and  in  the  course  of  the  vast  ages  of  geological 
time,  not  only  hills,  but  mountains  and  volcanoes,  have  been 
destroyed  mainly  by  the  action  of  this  slow  melting  away  of 
the  rocks.  By  the  folding  and  elevation  of  the  strata,  new 
tasks  are  constantly  set  before  these  agents,  and  we  may 


DENUDATION   OF  THE  LAND, 


237 


say  that  there  are  two  opposing  forces  at  work,  one  tending 
to  increase  land  elevations,  the  other  to  lower  them.  In  this 
combat,  elevation  has  excelled ;  and  as  a  result  we  have  a  very- 
irregular  land  surface.  If  there  had  been  no  weathering, 
the  land  elevation  would  have  been  vastly  greater,  but  the 
surface  of  the  land  would  have  been  much  more  regular. 
If  there  had  been  but  one  elevation,  and  that  at  the  begin- 
ing,  the  land  would  have  been  worn  down  to  a  nearly 
level  plain. 

Had  weather- 
ing been  the 
only  agent  of 
destruction,  the 
result  would 
have  been  very 
different.  With 
nothing  to  re- 
move the  frag- 
ments, the  solid 
rock  would 
have_  Jjii«r  cov- 
ered with  a  soil 
that  would  have 
protected      the 

strata  from  further  destruction ;  and  the  longer  it  acted,  the 
less  its  power  would  be,  the  process  being,  as  it  were,  self- 
destructive.  There  have  been  other  agents  at  work,  and 
these  have  served  to  remove  the  disintegrated  rock  frag- 
ments. Some  of  these  agents,  being  chemical,  have  carried 
the  material  away  in  solution,  others  have  acted  mechani- 
cally. These  are  described  under  the  following  heading 
of  erosion. 

Among  the  results  of  weathering,  one  of  prime  importance 


Fig.  119. 
Disintegrated  rock,  forming  residual  soil. 


238 


PHYSICAL   GEOGRAPHY. 


to  man  is  the  formation  of  soil.  In  many  parts  of  the  earth 
the  soil  is  the  result  of  rock  disintegration  (Fig.  119);  and 
in  some  places,  particularly  in  the  tropics,  this  residual  soil 
(so  called  because  it  is  largely  composed  of  the  insoluble 
residue  of  rock  decay)  has  a  depth  of  100  or  200  feet. 
In  this  country  it  is  of  particular  importance  in  the  Southern 
States,  the  soil  of  the  Northern  States  being  largely  the  result 
of   glacial  action,  and  being   a  transported  soil.     Another 

important  effect  of 
this  rock  decay,  is 
that  it  furnishes  to 
rivers  the  larger 
part  of  the  sediment 
load  Avith  which  they 
are  able  to  cut  their 
channels,  the  rock 
particles  being  used 
as  cutting  tools. 

Agents  of  Erosion. 
—  In  certain  places, 
various  agents  are 
at  work  cutting  into 
the  rocks  and  re- 
moving materials,  either  chemically,  mechanically,  or  both. 
The  most  important  of  these  are  wind,  rain,  percolating 
water,  rivers,  oceans,  and  glaciers. 

Wind  Erosion.  —  In  some  places  the  action  of  the  wind 
is  of  considerable  importance ;  but  in  most  regions  a  forest 
or  grass  covering  protects  the  rock  and  soil  from  its  action. 
On  the  seashore  the  blowing  of  the  wind  drives  sand  about, 
and  with  it  often  batters  the  rocks  in  a  manner  analogous  to 
the  sand  blast  with  which  glass  is  ground.  On  some  of  the 
sandy  islands  of  the  seacoast,  the  window  panes  are  some- 


FiG.  120.     • 
Sand  dunes,  Cape  Ann,  Mass. 


DENUDATION  OF  THE  LAND.  239 

times  transformed  to  ground  glass.  Many  narrow  islands 
along  the  seashore  are  built  above  sea  level  by  the  action 
of  the  Avind  upon  the  sand,  which  is  washed  into  the  form  of 
bars  by  the  waves ;  and  on  some  coasts  this  sand  is  driven 
inland,  where  it  accumulates  as  hills,  known  as  sand  dunes 
(Fig.  120).  In  the  arid  regions,  where  the  soil  is  not 
covered  with  dense  vegetation  (Fig.  121),  the  winds  are 
constantly  engaged  in  the  removal  of  the  finer  rock  frag- 
ments ;  and  in  these  places  the  wind  becomes  one  of  the 
most  important  agents  of  erosion.  Oftentimes  the  air  is  filled 
with  blown  sand,  so  that  even  neighboring  hills  are  obscured. 
This  natural  sand  blast 
beats  against  the  rocks, 
and  wears  them  away,  re- 
moving all  the  finer  par- 
ticles as  fast  as  they  fall 
from  the  rocks  (Figs.  69 
and  121). 

JRain  Ei'osion.  • —  During 
a  rain,  the  drops  that  reach  Fig.  121. 

the  soil  do  a  slight  amount     Moqui  Pueblo,  New  Mexico,  a  rocky  point 
p  .  ,    ,  ,  exposed  to  wind  action. 

01  erosion  and  transporta- 
tion, particularly  if  they  fall  upon  a  hillside.  Even  before 
the  rain  gathers  into  little  rills,  it  does  some  work  of  this 
kind  ;  and  when  it  has  formed  tiny  streams,  it  commences  to 
wash  the  soil  down  toward  the  rivers.  This  is  one  of  the 
ways  in  which  rivers  are  supplied  with  their  load  of  sediment. 
During  a  rain,  one  may  see  this  process  upon  a  plowed  field 
or  on  a  road.  In  the  forest,  and  upon  turf -covered  land,  this 
action  of  the  rain  is  of  little  importance ;  but  in  dry  regions, 
where  the  soil  is  not  protected,  every  rain  causes  the  soil  to 
creep  down  the  hillsides ;  and  in  the  mountains  of  the  arid 
regions,  great  gravel-slopes  are  by  this  means  accumulated  at 


240 


PHYSICAL   GEOGRAPHY. 


the  mountain  bases.  This  form  of  erosion  merges  into  that 
of  rivers.  In  some  places  (Plates  20,  21,  and  29)  rain  erosion 
has  carved  the  soft  clay  of  the  arid  lands  into  a  series  of  fan- 
tastic and  remarkable  forms. 

Gravity  is  an  important  factor  in  this  and  other  kinds  of 
erosion ;    but  even  when  unaided  by  any  of  the  agents  of 


Fig.  122. 
River  receiving  the  load  from  a  talus  at  the  base  of  a  canon  "vrall. 

erosion,  gravity  alone  is  in  some  places  an  agent  of  destruc- 
tion. The  fragments  loosened  from  cliffs  by  frost,  or  other 
agents  of  weathering,  fall  to  their  base  and  accumulate  there 
as  talus  slopes  (Figs.  118, 122,  and  219).  This  is  an  impor- 
tant source  of  sediment  for  rivers,  and  among  mountains,  the 
talus  slopes  are  important  elements  in  the  topography. 

Percolating  Water.  —  A  second  part  of  the  rain  enters  the 


DENUDATION   OF  THE  LAND.  241 

ground;  and  aside  from  the  work  of  rock  destruction  de- 
scribed above,  it  does  an  important  work  of  rock  removal. 
This  is  largely  chemical,  but  partly  mechanical.  It  removes 
soluble  substances;  and  when  it  again  reaches  the  surface, 
some,  if  not  all  of  this,  is  furnished  to  streams  for  transporta- 
tion, and  thus  much  of  it  finds  its  way  to  the  sea. 

The  most  important  mechanical  work,  is  that  of  aiding 
the  sliding  of  the  soil  down  the  hill  slopes.  The  percolating 
water  makes  the  soil  particles  slippery,  and  in  some  cases 
great  masses  fall  down,  forming  avalanches  or  landslides. 
These  very  frequently  occur  where  a  porous  layer  rests  upon 
an  impervious  one,  as  for  instance  when  a  sand  stratum  rests 
upon  a  layer  of  clay.  The  clay  is  lubricated  and  a  slipping 
plane  produced ;  and  then  under  favorable  circumstances,  a 
mass  of  earth  falls  down.  A  strong  wind  blowing  through 
the  trees  may  start  the  slide,  or  the  action  of  frost,  or  of  a 
heavy  rain,  may  introduce  the  conditions  which  are  necessary 
for  the  beginning  of  the  landslide. 

River  Erosio7i.  —  The  subject  of  rivers  is  taken  up  in  the 
next  chapter,  and  only  a  few  words  need  to  be  given  to  it 
here.  The  river  is  engaged  in  three  great  tasks,  (1)  the 
removal  of  water  from  the  land,  (2)  the  transportation  of 
sediment  given  to  it,  and  (3)  the  cutting  of  its  channel. 
Two  kinds  of  material  are  furnished  to  it,  (1)  mineral  matter 
in  solution,  largely  supplied  by  the  underground  water  which 
is  tributary  to  the  stream,  and  (2)  fragments  of  rock  furnished 
by  weathering.  Under  different  circumstances,  the  amounts 
of  these  substances  vary  greatly.  Some  streams  are  clear 
and  free  from  sediment,  others  are  always  filled  with  mud ; 
but  most  streams  are  usually  clear,  and  become  clouded  with 
sediment  only  after  a  heavy  rain.  In  some  cases,  the  ma- 
terial carried  is  in  the  form  of  fine  mud ;  in  others  it  is 
pebbles  and  even  large  boulders  (Fig.  124).     All  streams 


242 


PHYSICAL   GEOGFAPHY. 


carry  substances  in  solution,  but  some  have  a  little,  while 
others  carry  great  quantities  ;  and  in  desert  regions,  the 
rivers  are  sometimes  so  full  of  dissolved  substances,  that 
the  water  tastes  bitter  or  salt. 

Armed  with  its  load  of  sediment,  the  river  cuts  the  rocks 
of  its  channel,  and  deepens  its  valley ;  and  by  swinging  from 

one  side  to  the 
other,  it  broadens 
the  valley  slight- 
ly. Thus  by  river 
erosion,  there  is 
produced  a  rela- 
tively deep  and 
narrow  channel,  a 
gorge,  or  a  canon. 
In  arid  regions, 
where  weathering 
is  of  little  impor- 
tance, this  is  the 
prevailing  type  of 
river  valley ;  but 
most  of  the  val- 
leys of  moist  coun- 
tries are  U-shaped 
rather  than  V- 
shaped.  This  is  because  the  action  of  weathering  has  caused 
the  valley  sides  to  melt  back.  River  erosion  deepens,  weath- 
ering broadens  the  valleys  (Fig.  123)  ;  and  since  the  latter 
acts  more  slowly  than  the  former,  when  streams  begin  their 
work,  they  produce  deep,  narrow  valleys,  even  in  moist 
countries.  They  cut  down  much  more  rapidly  than  weath- 
ering can  broaden,  and  hence  young  valleys  are  gorges  ;  and 
this  is  true  wherever  erosion  greatly  exceeds  weathering. 


Fig.  123. 

Yellowstone  Valley,  showing  the  broadening  of  a 
V-shaped  valley  by  weathering. 


DENUDATlOl^   OF  THE  LAND. 


243 


The  rate  of  erosion  varies  with  the  slope  and  the  volume 
of  water  in  the  stream.  Where  the  slope  is  great,  if  other 
conditions  are  favorable,  the  erosion  is  rapid  ;  and  where  the 
amount  of  water  is  great,  the  erosion  is  more  rapid  than 
under  similar  circumstances  with  smaller  volume.  There- 
fore in  the  same  stream,  the  amount  of  erosion  done  during 
its  swollen  condition,  greatly  exceeds  that  done  when  the 
amount  of  water  is  not  great  (Fig.  124). 


Fig.  124. 

Westfield  River,  Massachusetts,  showing  boulders  which  may  be  moved 

when  the  river  is  swollen. 


The  rate  also  varies  with  the  amount  of  sediment;  for  if 
there  is  no  sediment,  there  are  no  tools  with  which  to  work, 
and  clear  water  can  do  little  work  except  that  of  solution, 
which  is  relatively  unimportant.  On  the  other  hand,  if 
the  river  is  given  more  sediment  than  it  can  dispose  of,  it 
cannot  cut  its  channel,  but  must  deposit  some  of  its  load  in 
the  valley,  as  is  being  done  in  the  lower  Mississippi.  The 
most  favorable  condition  is  that  of  a  moderate  amount  of 


244  PHYSICAL   GEOGRAPHY. 

sediment.  With  the  hardness  of  the  rocks  there  is  also  a 
variation;  for  a  river  cannot  cut  its  channel  so  rapidly  in 
a  hard  granite  as  it  can  in  a  soft  clay. 

From  this  it  will  be  seen,  that  the  rate  and  kind  of  work 
that  a  stream  is  doing,  varies  greatly  according  to  circum- 
stances ;  and  it  follows  that  river  valleys  must  present  very 
different  characteristics.  Some  are  narrow,  others  broad ; 
some  deep,  others  shallow ;  some  have  rapid  slope,  others 
have  a  gentle  flow,  etc.     In  carving  the  land,  river  erosion 


Jb'iu.  125. 
An  oceanic  volcanic  island,  showing  a  cliff  produced  by  wave  action  in  eating 

back  into  the  land. 

is  an  important  agent ;  but  its  importance  does  not  depend 
so  much  upon  the  work  of  cutting  it  does,  as  upon  the  fact 
that  it  is  the  agency  by  which  rock  fragments,  prepared  by 
other  means,  are  removed  from  the  land.  River  erosion  and 
weathering  are  intimately  combined  in  the  destruction  of 
the  land,  and  in  the  sculpturing  of  its  surface. 

Ocean  Erosion.  —  The  action  of  the  ocean  in  eroding,  is 
confined  to  the  limited  area  of  the  immediate  coast  line ;  but 
here  it  is  often  very  important.     The  waves  are  constantly 


DENUDATION  OF  THE  LAND.  245 

beating  on  the  shore,  and  battermg  at  the  rocks,  often  with 
terrific  force.  Armed  with  sand  and  pebbles,  and  even  by 
its  direct  action,  the  wave  is  able  to  wear  back  even  the 
hardest  rocks;  and  in  the  ocean,  islands  that  were  once  of 
great  size  are  now  only  remnants  (Figs.  125  and  195). 

On  the  beaches  and  on  the  headlands,  rocks  are  being 
ground  into  finer  particles.  The  materials  thus  obtained, 
added  to  those  received  from  other  sources,  are  removed, 
mainly  by  the  movements  of  the  wind  and  tidal  currents, 


Fig.  126. 
A  granite  hill  rounded  by  glacial  action. 

and  distributed  over  the  bottom  of  the  sea  near  the  land.  In 
these  ways  coasts  are  changed  in  form,  and  are  ever  changing ; 
though  here,  as  in  most  other  geological  changes,  the  work 
is  slowly  accomplished.  On  the  British  coast,  where  the 
changes  have  been  studied  for  centuries,  it  is  found  that  the 
coast  line  has  been  very  decidedly  altered  by  ocean  erosion. 

Glacial  Erosion.  —  Glaciers  are  now  relatively  scarce  in 
this  country,  but  at  one  time  they  were  present  in  northern 
United  States  (Chapter  XVII.),  and  they  then  did  consider- 
able work  of  erosion.     Because  it  is  a  rigid  body,  ice  acts  dif- 


246  PHYSICAL   GEOGRAPHY. 

ferently  from  water.  There  is  no  cliemical  work  done,  and 
the  mechanical  work  is  different ;  for  the  ice  exerts  a  great 
pressure,  and,  armed  with  rock  fragments,  it  scours  its  bed 
in  a  manner  analogous  to  a  great  sandpaper.  It  rounds  off 
the  surface  (Fig.  126)  and  acts  all  over  its  bed,  so  that  if  it 
spreads  over  a  country,  it  scours  hills  as  well  as  valleys. 
Mountain  glaciers  move  down  the  valleys,  scouring  their 
bottoms  and  sides,  and  transporting  much  rock  material. 

Denudation.  —  The  combined  action  of  these  forces  of 
weathering  and  erosion  is  denudation.  In  intimate  relation 
they  all  act  toward  the  one  end  of  reducing  the  land  ;  and 
in  this  respect  they  are  in  opposition  to  the  great  internal 
force  which  is  causing  the  land  to  rise  and  fall.  They  owe 
their  power  mainly  to  forces  from  without  the  earth.  The 
moon  and  sun  produce  the  tides,  the  sun  causes  the  changes 
in  the  weather,  the  atmosphere  acts  as  the  intermediary,  the 
ocean  furnishes  the  water,  and  two  internal  forces  furnish 
the  opportunity,  —  internal  heat  and  gravity.  The  former 
gives  elevations  to  be  destroyed,  the  latter  draws  the  water 
to  the  earth  and  causes  a  tendency  for  the  materials  to  move 
from  higher  to  lower  places.  Weathering  is  the  great  agency 
of  preparation;  for  their  chief  work,  the  erosive  agents  do 
some  destruction  and  much  transportation;  and  the  ocean, 
aside  from  its  work  of  erosion,  is  the  great  receiving  ground 
for  the  waste  from  the  land. 

These  changes  are  in  progress  at  all  times,  and  they  have 
been  so  through  all  of  the  geological  ages,  with  the  result 
that,  although  slowly  acting,  they  have  produced  enormous 
changes.  The  present  land  forms  are  the  result  of  the  action 
of  these  forces  (Plate  21  illustrates  exceptionally  rapid 
denudation)  ;  and  since  they  are  still  acting  as  in  the  past, 
the  surface  of  the  earth  is  even  now  changing.  The  land  is 
therefore  in  one  stage  of  its  history,  and  we  must  not  look 


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248  PHYSICAL   GEOGRAPHY, 

upon  the  hills  and  valleys  as  unchanging  and  unchangeable, 
but  rather  as  things  of  life,  with  a  past  history  to  be  read, 
and  a  future  to  be  predicted. 


-•o»- 


REFERENCE   BOOKS. 

Aside  from  those  to  which  reference  has  been  made  at  the  close  of  the 
preceding  chapter :  — 

Lyell.  — Principles  of  Geology,  Vols.  I.  and  II.  Appleton  &  Co.,  New  York. 
Eleventh  edition,  1872.  8vo.  $8.00.  (This  is  the  great  geological  classic, 
especially  complete  on  the  subject  of  denudation.) 

Shaler.  —  First  Book  in  Geology.  Heath  &  Co.,  Boston,  1884.  12mo. 
$1.00.     (This  interesting  little  book  is  written  for  beginners.) 

Shaler.  — Aspects  of  the  Earth.  Scribner,  New  York,  1889.  8vo.  $2.50. 
(Several  chapters  on  topics  touched  upon  in  this  and  the  preceding 
chapter.) 

For  Soils,  see  article  by  Shaler,  "  Twelfth  Annual  KeportU.  S.  Geological 
Survey."     Washington,  D.C.,  1891. 

For  Artesian  Wells,  see  article  by  Chamberlin  in  the  Fifth  Annual 
Report  of  the  same,  1885. 

For  importance  of  Earthworms,  see  Darwin,  "The  Formation  of  Vege- 
table Mould."  Appleton  &  Co.  (International  Scientific  Series),  New  York, 
1883.     12mo.     $1.50. 

One  of  the  most  important  contributions  to  denudation  is  Gilbert's 
*'  Geology  of  the  Henry  Mountains,"  Washington,  1887.  (To  be  obtained 
only  from  the  second-hand  bookstores.) 

Many  valuable  and  interesting  papers  appear  in  the  regular  geological 
periodicals,  of  which  there  are  three  issued  in  this  country,  as  follows  : 
(1)  "Bulletin  of  the  Geological  Society  of  America."  Six  volumes  already 
issued  at  f  5.00  (to  libraries)  a  volume.  Address  Professor  H.  L.  Fairchild, 
Rochester,  New  York.  (2)  "American  Geologist,"  Minneapolis,  Minnesota, 
now  in  its  sixteenth  volume,  two  being  published  each  year.  Price  $3.50  a 
year.  (3)  "Journal  of  Geology,"  Chicago,  Illinois,  now  in  its  third  volume* 
Price,  $3.00  a  volume. 


CHAPTER  XIV. 

TOPOGRAPHIC  FEATURES  OF  THE  EARTH'S  SURFACE. 

Continents  and  Ocean  Basins.  —  The  surface  of  the  earth  is 
broken  by  a  series  of  great  irregularities,  forming  the  conti- 
nents and  the  ocean  basins.  There  are  two  groups  of  con- 
tinents, with  intermediate  basins  filled  with  water.  The 
continent  masses,  which  may  be  called  the  eastern  and  the 
western,  are  mainly  grouped  about  the  north  pole,  causing 
the  northern  to  be  the  land  hemisphere ;  and  the  oceans  are 
gathered  around  the  south  pole,  entirely  surrounding  it,  and 
extending  rather  triangular  tongues  northward,  toward  the 
north  pole.  The  two  sets  of  continents  are  themselves 
more  or  less  completely  divided  along  nearly  east  and  west 
lines.  This  division  is  north  of  the  equator,  and  it  is  the 
cause  for  the  separation  of  North  and  South  America,  and  of 
Europe  and  Africa.  With  these  partial  or  complete  oceanic 
separations  we  have  four  great  continent  masses:  North 
America,  South  America,  Africa,  and  Eurasia.  Australia, 
the  fifth,  is  somewhat  aberrant. 

The  oceans  are  developed  into  two  great  basins,  the  At- 
lantic and  the  Pacific,  the  latter  having  an  area  of  fully 
62,000,000  square  miles,  which  is  equal  to  nearly  one-third  of 
the  area  of  the  earth's  surface.  Besides  these,  there  are  the 
Arctic,  Antarctic,  and  Indian  oceans,  which  are  only  partially 
separated  from  the  others.  The  Atlantic  has  an  average 
breadth  of  a  little  less  than  3000  miles,  while  the  breadth  of 
the  Pacific  is  fully  twice  as  great  as  this.     And  we  find  the 

249 


250  PHYSICAL   GEOGBAPHT. 

same  difference  in  size  between  the  eastern  and  the  western 
group  of  continents.  The  American  continents  have  an 
average  breadth  of  but  little  more  than  2000  miles,  while 
the  average  breadth  of  Europe  and  Asia  combined,  is  over 
6000  miles. 

As  has  been  described  in  Chapter  IX.,  the  oceans  for  the 
most  part  consist  of  great  submarine  plains  or  plateaus,  here 
and  there  broken  by  gently  rising  ridges,  or  occasionally  by 
steeply  rising  volcanoes  or  sharp  mountain  ridges.     The  pre- 


FiG.  127. 
Relief  map  of  Eurasia  (Lambert's  projection). 

vailing  feature  of  the  ocean  bottom  is  that  of  uniform  level- 
ness ;  and  the  average  depth  of  this  great  submarine  plateau 
is  nearly  three  miles,  while  in  some  places  the  depth  is  over 
five  miles.  This  great  area,  which  is  about  three-fourths  that 
of  the  earth's  surface,  is  rendered  level  by  means  of  the  oceanic 
water  which  fills  the  basin.  Above  the  ocean  surface  the 
continents  rise  with  considerable  uniformity,  but  their  aver- 
age elevation  is  very  much  less  than  the  depth  of  the  ocean. 
The  average  elevation  of   the  land  surface  of  the  globe  is 


N 


TOPOGRAPHIC  FEATURES   OF  EARTH'S   SURFACE,       251 


S2 


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CD 
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Miss.B. 


Appalach- 
ians 

Atlantic 


about  2000  feet ;  and  it  is  only 
here  and  there,  along  mountain 
chains  and  plateaus,  that  greater 

elevations   are   found;   but   the  ^^^m  Pacific 

average  depth  of  the  ocean  is 
fully  six  times  as  much.  This 
difference  between  land  eleva- 
tion and  ocean  depression,  is  §  ^^^^^  Colorado 
shown  in  Fig.  128,  which  repre- 
sents a  cross-section  of  North 
America  and  the  Atlantic 
Ocean,  drawn  upon  the  same 
vertical  scale,  which  is  greatly 
exaggerated. 

Examining  the  continents  in 
a  little  more  detail,  we  find  that 
they  consist  of  plateaus  and 
plains  as  the  most  prominent 
features.  Usually  there  are 
two  plateau  areas,  one  upon 
either  margin  of  the  continent ; 
and  above  these  rise  more  or 
less  continuous  ridges,  which 
we  know  as  mountain  chains. 
This  feature  of  continents  is 
well  illustrated  in  North  Amer- 
ica (Fig.  129),  which  may  be 

considered  a  typical  continent ;  ^^H  Spain 

and  it  will  be  described  in  more 
detail  in  later  parts  of  the  chap- 
ter. Plains  usually  occupy  the 
interior  portion  of  the  continent, 
and  these  are  sometimes  in  the  form  of  low  plateaus ;   while 


Mid 

Atlantic 


252 


PHYSICAL   GEOGRAPHY. 


in  some  cases  they  are  even  interior  basins.    The  land  surface 
is  very  irregular,  the  irregularities  being  partly  due  to  origi- 
nal features  of  the  earth's  crust,  and  partly  to  the  sculpturing 
of  these  by  the  agents  of  denudation  (see  Chapter  XIII.). 
Geological    conditions    conclusively  prove   that   the  con- 


FiG.  129. 
Relief  map  of  North  America  (Lambert's  projection). 


tinents  are  subject  to  changes,  and  that  the  present  form  is 
merely  the  result  of  an  evolution  which  has  long  been  in 
progress.  Even  at  present,  in  some  cases,  there  are  changes 
of  considerable  moment  still  in  progress.  The  mountains 
which  form  the  border  of  the  continents  have  been  elevated 


-■r,r 


■I 


105       Longitude      101  West 


Face  page  253. 


FJ.A 


from  93      Greenwich        S9 


Jt.D.Si-rvosa.Tf.T. 


TOPOGBAPHIC  FEATUBES  OF  EABTS'S  SUBFACE,       253 

by  successive  foldings  of  the  rocks ;  the  plateaus  have  been 
produced  by  great  elevations  of  the  land ;  and  many  of  the 
plains  have  been  caused  by  the  filling  of  seas  from  the  waste 
of  the  mountains.  Many  forces  have  cooperated  to  build  the 
continents,  but  this  subject  is  properly  one  of  pure  geology. 
From  the  present  standpoint,  it  is  enough  to  know  that  these 
changes  are  going  on,  and  to  recognize  the  fact  that  the 
continents  are  not  of  the  same  form  at  all  times.  Indeed, 
there  seems  good  reason  for  believing  that  the  true  Ameri- 
can continent  does  not  end  at  the  present  shore  line,  but 
that  its  proper  boundary  is  along  the  margin  of  the  conti- 
nental shelf,  which  on  the  northeastern  side,  extends  to  a 
distance  of  from  50  to  100  miles  from  the  present  shore. 
At  this  point  the  great  ocean  abysses  commence,  and  from 
the  land  to  this  point  there  is  very  little  depth  to  the  ocean. 
(See  Fig.  128.) 

Physical  Geography  of  the  United  States.  —  The  best  way 
to  illustrate  the  typical  features  of  the  earth's  surface,  is  not 
to  make  a  hasty  survey  of  the  entire  surface,  but  rather  to 
consider  a  single  area  in  some  detail.  Thus  we  may  select 
the  United  States  (Plate  22)  as  a  typical  part  of  the  con- 
tinent, and  by  examining  the  physical  geography  of  this  area, 
form  an  idea  concerning  the  main  features  of  the  earth's  sur- 
face ;  for  we  have  very  nearly  every  important  topographic 
form  represented  within  the  boundaries  of  this  country.  For 
the  sake  of  completeness,  it  will  be  well  to  extend  the  boun- 
daries of  the  area  described,  for  a  short  distance  beyond  the 
Canadian  boundary,  in  order  to  include  a  portion  of  the  con- 
tinent which  is  essential  to  its  proper  consideration.  This 
area,  including  the  United  States  and  southern  Canada, 
forms  a  true  section  of  a  typical  continent.  We  may  divide 
this  area  into  five  great  divisions,  each  having  characteristic 
geographic  features.     These  are  the  Atlantic  Coast  Province, 


254  PHYSICAL   GEOGBAPHY. 

the  Eastern  Mountain  Ranges,  the  Canadian  Highlands,  the 
Mississippi  Valley  Plains,  and  the  Cordilleras  of  the  West. 

Atlantic  Coast  Area.  —  This  properly  includes  the  conti- 
nental shelf,  which  is  now  submerged  beneath  the  sea,  but 
which  is  a  submarine  plain  bordering  the  continent  and 
appearing  to  form  a  true  part  of  it.  Above  the  sea  level, 
the  continuation  of  this  area  is  represented  by  a  narrow  strip 
of  level  country  with  an  elevation  of  but  a  few  feet  above  sea 
level,  and  extending  from  New  Jersey  to  the  Rio  Grande. 
It  forms  a  low  plain  which  is  nearly  featureless,  and  which 
in  some  parts  is  in  the  condition  of  a  swamp.  It  is  but  a  few 
miles  in  width  in  the  northern  portion,  and  varies  in  width 
as  we  proceed  southward,  but  gradually  increases  until  the 
Gulf  States  are  reached.  A  large  part  of  Florida  is  included 
within  the  area,  and  along  the  Mississippi  valley  the  coastal 
plains  expand  and  extend  inland  to  a  considerable  distance. 
The  present  delta  and  floodplains  of  Mississippi,  Louisiana, 
and  a  part  of  Arkansas,  belong  to  this  coastal  area ;  and  in 
Texas  there  is  a  strip  whose  width  is  often  as  great  as  50  miles. 

On  the  landward  side  of  this  low-l^ng  plain,  is  a  more 
elevated  area  of  level  country,  which  is  also  a  true  plain,  but 
which  is  more  ancient  in  origin.  The  low  swampy  plains 
are  scarcely  drained ;  but  these  higher,  inland  plains,  are  cut 
by  river  valleys,  and  in  some  cases  carved  into  a  series  of 
rounded  hills.  For  the  most  part,  the  low  swampy  plains 
near  the  coast-line  are  of  little  use  to  man,  their  swampiness 
prohibiting  their  occupation,  though  this  does  not  apply  to 
some  parts  of  the  plains,  such  as  the  delta  and  floodplain 
region  of  the  Mississippi.  The  higher  plains  on  the  land- 
ward side  of  these,  are  much  better  adapted  to  occupation, 
and  it  is  upon  these  that  the  greater  part  of  the  agriculture 
of  the  Southern  and  Gulf  States  is  carried  on. 

The  Eastern  Mountains.  —  In  nearly  all  cases  mountain 


TOPOGRAPHIC  FEATURES   OF  EARTH'S   SURFACE.       255 

chains  are  found  rising  above  basal  plateaus.  This  is  true 
for  the  great  system  of  eastern  mountains,  the  Appalachians. 
Both  on  the  eastern  and  western  sides  of  these  chains,  there 
is  a  highland  country  which  is  a  true  plateau,  though  in 
most  cases  deeply  carved  by  stream  valleys.  There  are  two 
parts  to  this  system  of  eastern  mountains,  one  much  older 
than  the  other,  and  both  considerably  destroyed  by  the 
sculpturing  action  of  the  agents  of  denudation.  The  oldest 
series  of  mountains  date  back  to  the  first  beginning  of  the 
known  history  of  the  North  American  continent,  when  tJiey 
were  formed  as  very  high  mountain  chains.  A  considerable 
part  of  New  England  is  included  within  this  area  of  ancient 
mountains,  and  the  chains  extend  southward  through  the 
hills  of  NeAV  Jersey,  and  thence  along  the  eastern  base  of  the 
modern  Appalachians  into  the  Carolinas.  In  many  places 
these  would  not  be  recognized  as  mountains,  but  are  now  in 
the  form  of  low  hills.  They  have  been  worn  down  to  their 
very  roots,  and  nothing  but  hills  are  left  where  once  existed 
very  lofty  chains.  The  highest  remnants  of  these  mountains 
are  found  in  New  England  and  North  Carolina. 

The  true  Appalachians  were  mucli  more  recently  formed; 
but  yet  they  are  among  the  ancient  mountains  of  the  conti- 
nent. For  a  long  period  of  time  they  also  have  been  exposed 
to  the  destructive  action  of  denudation,  so  that  their  original 
form  is  very  much  altered.  They  are  no  longer  high  chains, 
and  in  point  of  size  and  grandeur  bear  no  comparison  with 
such  recent  mountains  as  the  Rockies,  the  Andes,  or  the 
Alps.  Formerly  they  were  much  higher  than  now,  and 
probably  their  features  were  much  more  like  those  of  the 
grander  mountains  of  the  globe.  At  present  they  consist  of 
a  series  of  ridges  and  ranges,  extending  in  a  northeasterly 
direction,  usually  with  nearly  level  tops,  and  in  no  case  rising 
to  great  heights. 


256  PHYSICAL   GEOGRAPHY. 

The  highest  part  of  the  eastern  mountains  is  Mitchell's 
Peak  in  North  Carolina,  whose  elevation  is  6688  feet.  In 
these  mountains  there  are  vast  stores  of  coal,  building  stone, 
iron,  and  other  products  which  are  of  use  to  man. 

The  Canadian  Highlands.  —  These  are  another  ancient 
series  of  mountains,  once  much  more  extensive  than  now, 
and  they  enter  this  country  in  only  one  or  two  places.  The 
Adirondacks  may  be  considered  a  part  of  this  highland  area, 
and  the  same  holds  true  for  the  hilly  region  near  Lake 
Superior.  At  present  this  region  is  occupied  by  a  series  of 
low,  rather  rounded  hills,  never  rising  to  great  mountain 
heights,  and  rarely  being  over  a  mile  above  the  level  of 
the  sea.  Among  the  Adirondacks  the  highest  point  is  Mt. 
Marcy,  which  is  5379  feet  above  sea  level.  For  the  most 
part  this  hilly  region  is  of  little  value,  partly  because  it  is 
situated  far  in  the  north,  and  partly  because  it  is  composed 
of  rocks  that  do  not  favor  the  formation  of  even  slopes  and 
deep  soil.  There  are  considerable  areas  of  valuable  mineral 
materials,  mainly  iron  and  copper.  The  St.  Lawrence  valley 
forms  quite  another  province.  ^ 

The  Central  Plains.  —  Extending  from  the  western  base 
of  the  Appalachians  to  the  Mississippi,  there  is  a  great 
area  of  plains,  which  gradually  decrease  in  elevation  toward 
this  river.  From  the  Mississippi  westward,  the  plains  con- 
tinue until  the  base  of  the  Rocky  Mountains  is  reached ; 
and  here  also,  as  the  mountains  are  neared,  the  elevation 
gradually  becomes  higher.  At  the  base  of  each  of  these 
mountain  systems,  the  plains  have  become  transformed  to 
true  plateaus,  in  the  case  of  the  Appalachian  plateau  with 
an  elevation  of  1000  or  2000  feet,  and  of  the  Rocky  Moun- 
tains with  an  elevation  of  over  5000  feet. 

This  great  area  of  plains  is  not  everywhere  level  or  roll- 
ing, but   in   some  of   its   parts   is  broken   by  truly  moun- 


TOPOGRAPHIC  FEATURES  OF  EARTH'S  SURFACE.       257 

tainous  irregularities.  This  is  true,  for  instance,  in  Indian 
Territory,  in  Arkansas,  in  part  of  Missouri,  and  elsewhere. 
Aside  from  these  limited  areas  of  mountainous  character, 
there  are  other  regions  which  have  been  very  much  cut  and 
dissected  by  stream  action.  However,  the  general  condition 
of  these  plains  is  that  of  gently  undulating  country.  They 
form  the  great  farming  belt  of  the  continent,  and  also 
contain  deposits  of  valuable  minerals,  such  as  coal,  iron, 
petroleum,  building  stones,  etc.  This  area  of  plains  is 
equal  to  fully  one-fourth  of  the  total  area  of  the  country, 
and  the  elevation  is  generally  less  than  2000  feet,  while 
nearly  one-half  of  the  area  has  an  elevation  of  less  than 
1000  feet. 

Since  occupied  by  man,  the  greater  part  of  this  area  has 
been  free  from  timber.  In  the  plains  of  the  far  west  this 
is  due  to  the  fact  that  the  climate  is  dry;  but  among  the 
prairies  of  the  east  the  cause  is  less  easily  ascertained.  Some 
think  that,  because  of  its  compactness,  the  soil  was  unfavor- 
able, others  that  the  timber  has  been  burned  off  by  fires ; 
but  neithej^  theory  can  be  considered  proven. 

The  Cordilleran  Area.  —  This  is  the  most  complex  of  our 
geographical  areas,  and  perhaps  should  be  subdivided,  though 
for  our  general  purpose  it  may  be  considered  as  one  great 
area.  In  the  main  it  consists  of  a  great  plateau,  Avith  an 
average  elevation  of  over  a  mile  above  the  sea  level,  above 
which  rise  several  mountain  chains.  Commencing  on  the 
eastern  base  of  this  Cordilleran  region,  we  will  examine  it 
in  cross-section  until  the  Pacific  is  reached. 

A  high  plateau  reaches  to  the  very  base  of  the  Rocky 
Mountains,  which  then  rise  to  great  elevations,  not  only 
above  the  sea  level,  but  also  above  the  plateau  itself.  The 
highest  part  of  the  Rockies  is  in  Colorado,  in  which  state 
there  is  a  total  area  of  nearly  13,000  square  miles  with  an 

8 


258  PHYSICAL   GEOGBAPHY, 

elevation  greater  than  10,000  feet,  while  several  peaks  rise 
above  14,000  feet.  The  chains,  which  extend  northward 
and  southward,  are  of  varying  heights  and  differ  also  in 
extension.  There  is  not  one  mountain  chain,  but  a  series 
which  together  make  the  Rocky  Mountains.  They  pass 
entirely  across  the  United  States,  entering  Canada  on  the 
north  and  Mexico  on  the  south. 

West  of  these  mountains  is  a  region  of  interior  drainage, 
known  as  the  Great  Basin.  In  reality  there  are  numerous 
interior  basins,  some  of  which  combine  to  form  a  Great 
Basin  (Plate  23  and  Fig.  151),  while  others  exist  as  separate 
smaller  basins  of  interior  drainage.  The  basin  region  is  a 
great  plateau  area,  everywhere  above  sea  level,  and  usually 
more  than  a  mile  above  the  level  of  the  sea.  It  is  entirely 
surrounded  by  high  mountains,  and  the  interior  plateau  itself 
is  broken  by  ridges,  known  as  the  Basin  Ranges,  which 
extend  in  a  north  and  south  direction. 

Bordering  the  Great  Basin  on  the  west,  is  the  Sierra 
Nevada  range,  which  passes  in  a  nearly  north  and  south 
direction,  from  the  northern  part  of  California  to  the  southern 
border  of  the  country.^  It  is  a  high  mountain  region,  but 
its  average  elevation  is  less  than  that  of  the  Rockies. 

West  of  the  Sierras  is  a  great  valley,  which,  with  minor 
interruptions,  extends  from  northern  United  States  to  the 
Gulf  of  California,  which  is  really  a  part  of  this  valley. 
The  Death  Valley  of  Southern  California  is  a  part  of  this 
interior  depression,  and  here  we  have  illustrated  the  rather 
rare  feature  of  an  interior  basin  below  sea  level.  Death 
Valley,  which  is  175  miles  long,  in  one  place  is  at  least  225 
feet  below  the  level  of  the  sea.  The  Sacramento  valley 
is  also  a  part  of  this  same  depression. 

1  There  is  no  uniformity  in  the  usage  of  the  term  Sierra  Nevada,  and  the 
boundaries  of  the  range  are  vaguely  and  variously  drawn. 


TOPOGRAPHIC  FEATURES  OF  EARTH'S  SURFACE.       259 

West  of  this  valley,  and  rising  almost  out  of  the  Pacific, 
is  a  fourth  series  of  mountains,  the  Coast  Ranges,  which 
extend  from  Lower  California  to  the  northern  boundary  of 
the  United  States,  and  apparently  as  far  as  Alaska.  They 
are  rugged  mountains,  and  among  them  are  found  some  of 
the  highest  peaks  on  the  continent. 

These  mountains  of  the  Cordilleras  are  much  more  recent 
than  those  of  the  eastern  part  of  the  continent.  Many  of 
them  were  formed  in  the  Tertiary  period,  and  there  is  evi- 
dence that  some  of  them  are  still  growing.  It  is  as  a  result 
of  this  that  they  are  so  rugged  and  so  high ;  for  they  have 
not  been  long  enough  exposed  to  the  action  of  denudation  to 
be  reduced  to  low,  rounded  forms. 

No  mention  has  been  made  of  volcanoes,  for  the  reason 
that  within  the  borders  of  the  United  States,  outside  of 
Alaska,  there  are  none  known  to  be  active.  That  this  has 
not  always  been  the  case,  is  shown  by  the  vast  number  of 
volcanic  cones,  in  all  stages  of  destruction,  which  dot  the  Cor- 
dilleran  region.  There  are  thousands  of  these  (see  Chapter 
XX.);  and  on  every  hand,  the  evidence  is  conclusive  that  in 
very  recent  times  large  areas  have  been  deluged  in  lava  and 
ash  deposits.  Along  the  eastern  margin  of  the  country  there 
is  no  sign  of  recent  volcanic  activity,  although  during  the  time 
of  formation  of  the  higher  mountains,  volcanoes  did  exist. 

Aside  from  being  the  largest  geographic  zone  of  the 
country,  the  Cordilleran  region  contains  minerals  in  extraor- 
dinary variety  and  abundance.  It  is  the  great  precious 
metal  zone  of  the  earth,  and  from  it  is  produced  more  gold 
and  more  silver  than  is  supplied  by  any  other  nation. 

The  Drainage  of  the  Country.  —  Three  oceans  receive  the 
waters  that  fall  in  the  United  States.  The  accompanying 
map  (Plate  23)  shows  this  so  graphically  that  description 
may  be  omitted. 


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to 

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o 

OD 

OS 
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Pi 

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bo 
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"S 

Q 


TOPOGRAPHIC  FEATURES  OF'  EARTH'S  SURFACE.       261 

The  Shore  Line.  —  The  coast  of  North  America,  like  that 
of  several  other  continents,  encloses  a  land  area  which  is 
triangular  in  form,  the  apex  of  the  triangle  being  toward 
the  south  (Fig.  129).  There  is  much  variety  in  the  form 
of  the  coast ;  but  the  coast  line  is  by  no  means  so  broken 
as  that  of  Europe  (Fig.  127).  There  are  several  great 
promontories,  notably  on  the  east  coast,  and  many  minor 
irregularities  along  this  coast  line;  and  these  are  much 
more  prominent  in  the  northern  than  in  the  southern  part 
(Plate  22).  In  the  east,  islands  abound  along  the  coast  of. 
Maine,  and  near  Florida.  On  the  Pacific  coast  there  are 
very  few  islands,  excepting  those  which  begin  to  be  numer- 
ous at  the  northern  boundary  of  the  United  States,  and  in- 
crease in  abundance  toward  Alaska. 


REFERENCE   BOOKS. 

Whitney.  —  The  United  States.  Little,  Brown,  &  Co.,  Boston,  1889. 
8vo.  $3.00.  (In  the  main  reproduced  from  an  article  by  the  author  in 
the  Encyclopedia  Britannica.) 

Shaler.  —  The  United  States  of  America.  Appleton  &  Co.,  New  York, 
1894.  Two  volumes.  8vo.  !§10.00.  (Particularly  Vol.  I,  in  which  the 
relation  between  man  and  nature  is  pointed  out.) 

Shaler. — The  Story  op  Our  Continent.  Ginn  &  Co.,  Boston,  1891. 
12mo.    $0.75.     (Of  value  to  the  student  for  elementary  reading.) 

As  a  type  of  the  kind  of  book  needed  to  adequately  describe  the 
features  of  this  country,  attention  may  be  called  to  Geikie,  —  Scenery  of 
Scotland.    Macmillan  &  Co.,  New  York,  1887.    Second  edition.   8vo.    $3.50. 

For  known  elevations  of  points  in  the  United  States,  see  Gannett.  — • 
Dictionary  of  Altitudes,  etc.  Bulletin  5,  U.  S.  Geological  Survey, 
Washington,  1884.  8vo.  $0.20.  Second  edition  of  same,  revised,  Bulletin 
76,  1891.    8vo.    393  pages.     $0.25. 

For  average  elevation  of  the  states  and  country,  see  Gannett,  Thirteenth 
Annual  Report,  U.  S.  Geological  Survey,  Washington,  1893. 


CHAPTER   XV. 


RIVER   VALLEYS. 


General  Description.  —  A  river  is  a  natural  drainage  line 

on  the  land,  and  it  usu- 
ally occupies  a  valley 
which  has  certain  quite 
definite  characteristics. 
On  either  side  of  the 
river  are  the  valley  sides 
or  walls,  sometimes  ris- 
ing gently,  sometimes 
steeply :  at  times  to 
great  heights  (Fig. 
130),  and  again  with 
only  low  elevations.  In 
the  lowest  part  of  the 
valley,  and  generally 
near  its  middle  portion, 
the  river  flows,  usually 
with  a  meandering 
course.  In  most  cases 
the  river  is  immediately 
bounded  by  rather 
steeply  rising  hanks^oiAj 
a  few  feet  in  elevation, 
beyond  which  the  slope 
is  more  gentle,  and 
sometimes  even  a  plain, 
which  in  times  of  flood  is  covered  with  water  (Fig.  157). 

262 


Fig.  130. 
A  deep  mountain  valley. 


RIVER    VALLEYS. 


263 


Fig.  1;51. 
Stream  issuing  from  a  limestone  cave. 


Here  and  there,  at  irregular  intervals,  tributaries  enter  the 
main  stream,  and  these  themselves  branch  into  other  tribu- 
taries, until  very  often  there  is  finally  produced  a  branching 
network  of  minor  streams,  all  directly  or  indirectly  contribut- 
ing to  the  supply  of  water  in  the  main  stream.  This  princi- 
pal supply  often  comes 
from  springs,  and  some- 
times the  source  of  the 
river  is  a  spring  (Fig. 
131).  Together  these  form 
a  river  system;  and  such  a 
system  may  have  an  area 
of  only  a  few  miles,  or  it 
may  drain  an  area  of 
many  thousand  square 
miles.     The  line  or  plane 

which   separates   one    stream    or   system    of    streams   from 
another,  is  known  as  a  divide  or  tvater  parting. 

All  streams  have  these  general  characteristics ;  but  when 
their  valleys  are  examined  in  detail,  there  are  found  to  be 
many  differences,  not  merely  between  different  rivers,  but 
even  in  the  several  parts  of  the  same  river.  The  valley  of 
one  stream,  or  a  part  of  a  stream,  may  be  a  precipitous  gorge 
(Fig.  142),  or  it  may  have  very  gently  sloping  sides  (Fig.  132). 
In  some  rivers  there  are  floodplains  and  deltas,  in  others 
these  are  absent ;  and  in  some  cases  the  rock  walls  of  the 
valley  may  rise  directly  out  of  the  river.  In  some  there 
is  a  permanent  flow  of  water,  in  others  the  supply  is  inter- 
mittent, and  in  some  extreme  cases  water  flows  only  once 
or  twice  a  year.  In  most  river  systems  the  tributaries  are 
numerous,  but  in  some  cases  they  are  few  ;  and  while  some 
of  these  join  the  main  streams  at  a  high  angle,  in  many  cases 
they  enter  it  at  an  acute  angle.     There  are  many  reasons  for 


264 


PHYSICAL   GEOGRAPHY. 


these  differences  in  streams,  the  two  most  important  being 
the  position  and  kind  of  rock  in  which  they  occur,  and  the 
stage  in  development  which  they  have  reached. 

Only  a  very  few  years  ago,  river  valleys  were  believed  to 
have  been  formed  by  some  earth  convulsion,  or  some  un- 
usual force,  and  it  was  thought  that  rivers  occupied  them 


r 


Fig.  132. 
Brink  of  Niagara  Falls.    A  valley  having  very  gently  sloping  sides  above  the 

falls  and  precipitous  sides  below. 

merely  because  they  were  valleys  ready  made.  It  was 
believed  that  the  crust  of  the  earth  had  been  contorted  and 
fissured,  that  ocean  floods  had  swept  over  the  land,  and  that 
the  rivers  had  practically  no  share  in  the  formation  of  the 
valleys  which  they  occupied.  We  now  know  that  the  major- 
ity of  rivers  have  formed  their  own  valleys,  that  they  have 
formed  them  in  a  very  slow  way,  and  that  most  of  them  are 


BIVER   VALLEYS. 


265 


still  engaged  in 
the  work  of  val- 
ley carving.  If 
this  be  so,  then 
river  valleys 
have  had  a  de- 
velopment and 
a  history  ;  and 
in  this  history 
we  may  hope  to 
find  an  explana- 
tion of  many  of 
the  differences. 
Development 
of  River  Val- 
leys.— We  must 
bear     in     mind 


Fig.  134. 

Royal  Gorge,  Col.    A  mountain  gorge 

where  erosion  is  in  rapid  progress. 


Fig.  133.  A  gorge  near  Ithaca,  N.Y.,  illustrating  the 
down-cutting  of  a  stream  valley  and  its  broadening 
by  weathering. 

that  the  river  valley  is  the 
result  of  the  combined  action 
of  stream  erosion,  which  tends 
to  deepen  (Fig.  133),  weath- 
ering which  tends  to  broaden 
the  valleys  (Fig.  123),  and  the 
transportation  of  sediment  fur- 
nished by  these  means.  Ero- 
sion proceeds  more  rapidly 
than  weathering,  but  there 
comes  a  time  when  its  action 
is  checked.  In  no  part  of  the 
valley  can  the  stream  cut  be- 
low the  sea  level,  or  below 
base  level,  as  it  is  called ;  and 
since     it    must    carry    water 


266 


PHYSICAL   GEOGBAPHY. 


down  a  slope,  in  its  erosion  the  river  reaches  lower  levels 
near  its  mouth  than  higher  up  in  its  course.  Until  a 
line  of  easy  slope  is  reached,  erosion  exceeds  weathering 
(Fig.  134)  ;  but  then,  since  erosion  is  checked  while  weath- 
ering continues,  the  latter  produces  its  most  marked  effect, 
and  the  valley  gradually  broadens,  while  the  hills  slowly  melt 


Fig.  135. 
Oxbow  cut-off  in  Connecticut  Valley,  Northampton,  Mass. 

river  valley  with  rounded  valley  walls. 


A  broad  mature 


down.  Unless  interfered  with,  this  would  continue  until  the 
surface  was  base-leveled^  or  reduced  to  a  nearly  level  con- 
dition. 

Different  parts  of  the  river  will  work  at  different  rates  ; 
and  under  variable  conditions,  this  development  from  the 
canon  or  gorge-like  valley  of  youths  to  the  broad  valley  with 
rounded   sides    (Fig.    135),  which   characterizes   the   more 


BIVER   VALLEYS, 


267 


Fig.  13j. 

Development  of  the  canon.     A  and  C,  layers  of  hard 

rock,  B  and  D  soft. 


mature  stages,  may  proceed  at  different  rates  and  with  dif- 
ferent results.  Naturally  the  part  of  the  river  which  is 
nearest  the  mouth 
is  first  and  most 
easily  developed, 
for  it  is  nearest  the 
sea  level.  There- 
fore a  stream  val- 
ley may  have  the 
narrow  gorge-like 
condition  among 
its  head  waters, 
while  the  lower  portion  is  broadened  into  mature  form. 

A  part  of  the  stream 
may  flow  through  soft 
rocks,  and  another  por- 
tion through  hard  lay- 
ers ;  and  then,  even 
in  short  distances,  the 
form  of  the  valley  may 
change  ;  for  the  process 
of  broadening  by  weathering  is  much  more  rapid  in  the  soft 
than  in  the  hard  strata. 
A  stream  may  be  able  to 
cut  in  one  part  of  its  course, 


Fig.  137. 

Development  of  the  canon  profile  in  an  arid 

region.     1  and  3  hard,  2  and  4  soft  strata. 


■^  '<( 


^ 


A— - 


fifl 


,-e—— 


" ^^ — >.  'v     >^--^- 1 — -^ 7 


5L 


-Xii:-4Ju^-^ 


y-' 


ii^.ii-- 


and  be  obliged  to  deposit 
sediment  in  another  por- 
tion ;  and  in  the  latter 
case,  erosion  is  checked, 
while  weathering  continues 
to  produce  a  perceptible 
effect.  In  an  arid  climate,  Avhere  weathering  is  relatively 
unimportant,  the  valleys  are  almost  all  in  the  condition  of 


Fig.  138. 
Diagrammatic  representation  of  develop- 
ment  of    a   young  valley    («a)  to  old 
age  (M). 


268  PHTSICAL  GEOGRAPHY. 

the  gorge  or  canon  (Figs.  136,  137)  ;  and  among  moun- 
tains, where  the  elevation  and  slope  are  great,  erosion  so 
exceeds  weathering  that  the  gorge  is  the  characteristic 
valley  (Fig.  134). 

•  During  the  time  when  erosion  exceeds  weathering,  —  that 
is  during  youth,  —  the  resulting  valley  is  deep  and  relatively 
narrow  ;  and  wherever  we  see  this  kind  of  valley,  we  may 


Fig.  139. 
The  Yellowstone,  a  young  valley  broadening  by  weathering  and  being  deepened 

along  a  narrow  line  by  the  river  erosion. 

be  certain  that,  for  one  reason  or  another,  erosion  is  now,  or 
has  recently  been  in  progress.  That  weathering  is  also  pro- 
ducing an  effect,  is  evident  from  the  fact  that  the  valley  is 
wider  at  the  top  than  at  the  bottom,  because  the  former  has 
for  a  longer  time  been  exposed  to  its  action  (Fig.  139).  In 
such  cases  the  river  is  often  a  series  of  cascades  or  falls, 
because  (see  Chapter  XVI.)  in  its  rapid  down-cutting,  the 


BIVER   VALLEYS. 


269 


■<— : 


stream  finds  rocks  of  different  powers  of  resistance,  and 
therefore  cuts  its  bed  irregularly.  Therefore,  in  addition  to 
gorges,  waterfalls  characterize  youthfulness  in  river  valleys. 
In  many  cases  lakes  are  also  present ;  and  since  the  process 
of  lake  destruction  or  re- 
moval is  a  simple  and  brief 
task,  they  do  not  long  re- 
main in  the  river  valley. 

The  development  of  the 
stream  proceeds  most  rap- 
idly near  its  mouth,  and 
later  in  the  headwaters  ; 
and  consequently,  tribu- 
taries   are    not    numerous 

at    first    (Fig.     140)  ;     but    ^  ^'^^  of  drainage  in  Illinois,  showing  slight 

.  development  of  tributaries. 

one   by  one  they  begin  to 

develop,  until  all  of  the  area  is  brought  under  the  influence 
of  some  stream  or  rill  (Fig.  141).     At  first  the  divides  are 

not  very  definite,  and  they  may 
be  flat-topped  and  swampy;  but 
in  maturity  these  become  quite 
sharply  defined,  and  usually  every 
part  of  the  area  is  drained. 

When  vertical  erosion  has 
ceased,  the  work  of  the  river  be- 
comes merely  that  of  a  transporter 
of  sediment,  except  that  in  swing- 
ing about,  the  river  does  some 
lateral  erosion  on  its  banks.  The 
characteristics  of  youth  disappear, 
waterfalls  are  worn  down,  lakes 
are  filled  and  destroyed,  the  gorge  is  broadened  to  the  gently 
sloping  valley  side  (Fig.  138),  and  the  number  of  tributaries 


Fig.  141. 

A  bit  of  West  Virginia  drainage, 
illustrating  well  -  developed 
tributaries  of  maturity. 


270 


PHYSICAL   GEOGRAPHY. 


increases.  With  tlie  broadening  valley,  and  the  decrease  in 
river  slope,  the  conditions  favoring  floodplains  are  brought 
about ;  and  since  the  first  and  most  rapid  development  is  in 
the  lower  part  of  the  river,  in  this  stage  the  valley  may  con- 
sist of  three  quite  different  parts,  —  a  lower  flood-plained 
course,  a  middle  portion,  and  an  upper  torrential  part,  with 

gorges  and  waterfalls.  The 
majority  of  streams  have 
reached  this  stage,  and 
this  is  why,  in  describing 
a  river,  it  is  commonly  said 
that  it  consists  of  these 
three  parts ;  but  really 
this  is  to  be  considered  as 
merely  a  stage  in  develop- 
ment, to  reach  which  other 
stages  are  passed  through, 
and  which  is  normally  suc- 
ceeded by  others.  Since 
all  rivers  are  not  in  the 
same  stage  of  development, 
a  careful  examination  of 
the  valleys  of  a  country 
shows  many  exceptions  to 
this  condition  of  early  ma- 
turity. 
Naturally  there  is  much  difference  in  the  rate  of  develop- 
ment, and  in  the  result  produced  under  different  circum- 
stances. Whether  the  river  develops  in  a  mountain  or  on  a 
plain,  or  in  an  arid  or  humid  climate,  the  main  fact  is  the 
same,  —  that  there  is  this  development  from  immature  gorge 
to  broad  valley.  On  a  low  plain  near  the  sea  level,  the  rate 
of  development  in  the  soft  clay  is  much  more  rapid  than  in 


Fig.  142. 
Canon  of  the  Colorado. 


EIVER   VALLEYS, 


271 


a  high  plateau  ;  but  while  in  the  former  there  are  produced 
only  shallow  trenches  a  few  feet  in  depth,  in  the  latter  a 
canon  may  be  cut  with  a  depth  of  thousands  of  feet.  The 
former  we  see  in  the  plains  bordering  the  coast  of  Texas, 
the  latter  in  the  Colorado  canon  (Fig.  142).  In  the  hard 
rocks  of  the  Colorado  the  form  of  the  canon  is  preserved, 
and  this  is  also  favored  by  the  dry  climate ;  but  the  soft, 
clay  banks  of  the 


'vtf^lttif^ffiiVn^in^Mtttfifft-'fi'  f^vx- 


Texas  streams 
readily  crumble 
under  the  action 
of  weathering  in  a 
moist  climate.  The 
development  of  the 
latter  to  the  state 
of  maturity,  will 
therefore  be  much 
more  rapid  than 
that  of  the  former, 
—  just  as  some 
animals  or  plants 
pass  through  life 
in  a  few  weeks, 
while  others  live 
for  a  century. 

In  a  mountainous  country  the  elevation  is  so  great,  and 
the  rock  structure  so  complex,  that  gorges  will  remain  for 
long  periods  of  time  ;  and  ages  must  elapse  before  the 
erosive  action  of  the  river  becomes  less  rapid  than  weather- 
ing. Now  and  then  a  deep  mountain  lake  may  check  the 
work  of  the  river,  and  serve  as  a  temporary  base  level,  below 
which,  for  the  time  being,  the  stream  cannot  cut ;  and  so 
here,  for  a  short  distance,  the  valley  may  become  broadened. 


Fig.  143. 
A  broad  Alpine  valley. 


272 


PHYSICAL   GEOGRAPHY, 


While  the  prevailing  type  of  mountain  stream  valley  is 
that  of  the  gorge  (Fig.  134),  there  are  mountain  valleys  of 
great  breadth  and  depth.  These  are  not  true  stream  valleys, 
but  great  synclinal  valleys  of  rock  folding  (Fig.  143)  which 
the  rivers  have  occupied  because  of  their  convenient  location. 
After  passing  through  a  deep  defile  (Fig.  144),  a  tiny  stream 
may  emerge  into  one  of  these  great,  park-like  valleys  (Figs. 

143  and  221) ;  and 
then  we  see,  side 
by  side,  the  valley 
of  stream  forma- 
tion and  that  of 
rock  folding. 
With  the  aid  of 
weathering,  even 
the  mountain  gorge 
will  in  time  broad- 
en out  into  a  wide 
valley. 

Adjustment  of 
Streams.  —  When 
a  river  begins  to 
cut  its  valley  upon 
a  new  land,  there 
is  no  necessarv 
relation  between 
stream  course  and  rock  structure.  The  stream  may  flow 
across  hard  and  soft  layers  alike,  the  course  being  consequent 
on  the  topography,  because  the  river  was  guided  down  the 
original  slopes.  However,  as  the  river  develops,  it  often 
gradually  changes  its  course  in  order  to  follow  soft  layers 
of  rock;  and  therefore,  in  regions  where  the  rock  layers 
are  inclined,  many  river  courses  are  adjusted  to   the   rock 


Fig.  144. 
Mountain  gorge  in  tlie  Alps. 


BIVEB    VALLEYS. 


273 


structure,  soft  layers  being  the  site  of  valleys,  while  the  hard 
strata  stand  out  as  ridges.  This  is  characteristic  of  mature 
streams  which  have  had  a  long  period  of  development  and 
change. 

At  first  the  topography  guides  the  stream  course,  but 
finally  the  river  course  determines  the  topography.  In  such 
regions  as  New  England,  we  find  the  large  river  valleys 
cut  in  the  softer  beds  of  rock,  while  the  harder  strata  stand 
up  as  ridges.  Still,  here  and  elsewhere,  there  are  numerous 
exceptions  to  this  statement,  which  is  only  generally  true. 
This  mature  adjustment  is  well  shown  in  many  of  the  New 
England  and  Appalachian  streams.  Some  of  the  ways  in 
which  these  changes  take  place  are  described  in  the  next 
section. 

The  River  Divide.  —  Between  any  two  streams  there  is  a 
line,  or  an  area,  which  divides  the  waters,  sending  a  part  one 
way  and  the  rest  in  an  opposite  direction.  These  divides  or 
water  partings  are  by  no  means  permanent,  but  are  con- 
stantly and  usually  very  slowly 
changing.  The  stream  that  has  the 
most  power  pushes  the  divide  into 
the  territory  of  the  other,  and  there 
are  various  ways  in  which  one  stream 
may  have  more  power  than  another. 
One  may  have  a  shorter  course  to  the 
same  level,  and  hence  have  a  greater 
slope  (Fig.  145);  or  one  may  be 
cutting  through  soft  rock,  while  the 
opponent  is  working  in  hard  layers 
(Fig.  146) ;  or  (as  in  many  islands  in  the  trade-wind  belt) 
the  rainfall  on  one  side  of  the  divide  may  exceed  that  on  the 
other.  Gradually  the  divide  moves  into  the  area  of  the  stream 
having  the  least  rainfall,  or  the  least  slope,  or  the  hardest  rock. 


"■^ivr<^;^ 


Fig.  145. 


274 


PHYSICAL    GEOGRAPHY. 


Fig.  146. 


A  still  more  important  cause  for  the  change  of  divides  is 
found  among  tilted  rocks.  If  the  layers  of  a  series  of  strata 
stand  in  the  monoclinal  attitude,  and  if  these  alternate  in 

hardness,  the  soft  layers 
weather  more  rapidly 
than  those  which  are 
hard,  and  which,  because 
of  this  fact,  tend  to  re- 
main above  the  general 
level  (Fig.  261).  In 
such  a  case,  the  highest 
points  do  not  sink  verti- 
cally as  the  ridges  wear 
down;  but  they  move 
downward  and  back- 
ward in  the  direction 
of  the  dip  of  the  strata 
(Fig.  147).  This  is  so  permanent  a  condition  that  it  may 
be  stated  as  a  laAV,  that  in  rocks  of  monoclinal  attitude  the 
divide  migrates  in  the  direction  of  the  dip.  This  law  of 
7nonoclinal  shifting  applies  also  to  changes  in  river  courses. 
In  their  down-cutting,  the  valleys  also  tend  to  migrate  in 
that  direction,  and  this  is  one  of  the  reasons  why  streams 
adjust    themselves 


to  soft  layers  ;  for 
once  finding  them, 
they  tend  to  re- 
main in  them. 

Usually  the  mi- 
gration of  a  divide  is  an  extremely  slow  process,  and  in 
the  course  of  a  lifetime  one  would  not  notice  any  change ; 
but  under  exceptional  circumstances  it  may  become  rapid, 
and  in  a  brief  time  the  divide  may  change  for  many  miles. 


Fig.  147. 
Illustrating  monoclinal  shifting  of  divides. 


RIVER   VALLEYS, 


275 


This  will  happen  when  a  river  with  a  more  favorable  situa- 
tion, for  some  reason  gradually  pushes  its  divide  back  until 
it  taps  its  opponent.  Then  the  stream  receives  a  large  acces- 
sion of  drainage  area  and  carries  a  part  of  another  system 
across  the  old  divide  (Fig.  148).  Before  the  diversion,  the 
divide  was  low  and 
nearly  on  the  same 
level  as  the  stream 
about  to  have  its 
course  changed  ;  and 
then,  perhaps  during 
some  time  of  flood, 
the  new  course  was 
chosen  and  after- 
wards maintained. 
While  these  cases 
undoubtedly  occur, 
it  is  doubtful  if  they 
are  at  all  common  ;  and  the  ordinary  change  in  the  divide  is 
a  very  slow  one.  By  these  changes  in  divides,  the  adjust- 
ment of  streams  is  also  favored. 

Accidents  to  Streams.  —  River  valleys  tend  to  pass  through 
a  regular  cycle  of  development,  from  the  young  to  the  old 
stages  ;  and  if  nothing  intervened  to  prevent,  we  should  find 
them  all  in  some  stage  in  this  regular  cycle.  Some  would 
be  young,  others  mature,  and  others  old  ;  some  would  be 
upon  plains,  others  on  plateaus,  or  among  mountains.  There 
would  be  great  variety  in  river  valleys,  but  it  would  be  of 
a  regular  kind.  In  reality,  the  development  of  rivers  is 
subject  to  many  interruptions  of  various  kinds,  and  the  cycle 
is  never  entirely  passed  through  by  any  single  river.  The 
accidents  to  which  rivers  are  subjected,  sometimes  increase, 
sometimes  decrease,  the  power  of  the  stream.     In  the  course 


Fig.  148. 

Illustrating  sudden  shifting  of  a  divide  (aa)  to 
(bb)  by  carrying  the  headwater  (e)  across  the 
old  divide  at  (c) . 


276  PHYSICAL   GEOGRAPHY. 

of  its  development,  the  different  parts  of  a  river  may  experi- 
ence entirely  different  accidents,  and  the  resulting  valley 
will  be  complex  or  composite.  Any  single  part  of  a  stream 
may  also  suffer  a  variety  of  accidents. 

Land  Movements.  —  Land  movements  are  among  the  most 
common  accidents  which  interfere  with  normal  development ; 
and  these  are  of  three  kinds  :  (1)  broad  uplifts,  (2)  down- 
ward movements,  (3)  folding  which  accompanies  mountain 
formation.  With  the  general  uplift  of  a  country,  streams 
are  given  new  life,  or  rejuvenated,  and  we  may  then  have 
a  narrow  gorge  cut  in  the  center  of  a  broad  valley.  After  a 
long  period  of  denudation  the  uplift  gives  new  powers  to 

the  stream,  and  it 

then  cuts  a  nar- 

^ ___    row  valley  CFisr. 

:'.lC-'--i-.-'.-.'.J.':>--...LJ..l-.i,.!'\         /.C.l.^.l/j,U.J-!j...L-i.J.!i^-'.-^-U: '  -'  ./      V       o 


1,1,1,1,1,1  ,-L,^-r^-T^ 


I      I  '  I     I  '-r 


rrr-T 


n^i',!:^:i:i-rpp; 


|g;^;;^yg;^^^gag^g^^^  149).       Such    an 

uplift  may  affect 


Fig.  149. 

Diagram  showing  the  result  of    an  elevation,  which  gl'G^t  areas  ;    and. 

caused  the  inner  canon  of  the  Colorado  to  be  cut  be-  {n  ^J^g  rivcrs  thus 
tween  the  older  walls  of  the  outer  and  broader  valley.  .      _  „  .. 

revived,  waterralls 
again  begin  to  develop,  and  nearly  all  of  the  appearances 
of  youth  may  return.  Nearly  every  stream  system  shows 
some  sign  of  this  kind  of  rejuvenation,  which  has  affected  its 
recent  development.  If  this  elevation  happens  near  the  sea- 
coast,  a  part  of  the  ocean  bottom  is  raised  to  the  condition 
of  dry  land,  and  the  streams  of  the  old  mainland  extend 
across  it ;  and  perhaps  by  this  means  separate  streams  may 
be  united  to  form  one  system. 

Depression  of  the  land  would  rob  streams  of  some  of  their 
force  by  decreasing  their  elevation,  and  hence  their  slope. 
Along  the  coast,  the  lowering  of  the  land  causes  the  ocean 
to  extend  up  the  valleys,  drow7iing  parts  of  the  streams,  and 
transforming   their    mouths   to   estuaries    or   straits,   while 


riveh  valleys. 


277 


r.    1.1.  ipi,.  I JW* 

'        (^•fCanuU-n 


2/-^  Cope  Charles 


DELAWAKE 

And 

CHESAPEAKE  BAYS 

SCALE  OF  MILES 

I 1—1 ' 

0     5    10   15   20    25  30 

S.D.Sirtoti.lf.T. 


Plate  24. 
River  valleys  drowned  by  submergence  beneath  the  sea. 


278  PHYSICAL   GEOGRAPHY. 

numerous  islands  are  formed  where  the  hilltops  rise  above 
the  sea  (Figs.  193,  211  and  Plate  24).  This  entrance  of  the 
sea  produces  a  reverse  effect  from  that  of  elevation;  for 
the  lower  parts  of  streams  may  be  dissected.,  and  parts  of  one 
system  may  enter  the  ocean  through  separate  mouths.  This 
is  very  well  illustrated  in  many  cases  on  the  coast  of  Maine, 
and  particularly  well  in  the  Chesapeake  (Plate  24),  which, 
with  its  tributary  streams,  represents  a  part  of  a  river  system 
drowned  by  the  sea. 

When  the  strata  are  folded  in  the  form  of  mountains, 
stream  erosion  is  interfered  with  and  often  entirely  checked. 
As  the  mountains  rise,  a  dam  is  built  in  the  path  of  the 
rivers  ;  and  unless  their  rate  of  down-cutting  is  as  rapid  as 
the  rate  of  elevation,  which  in  most  cases  would  not  be  true, 
the  streams  will  suffer  interruptions.  If  they  persist  in 
their  course,  and  cut  their  channels  as  rapidly  as  the  moun- 
tains rise,  they  are  known  as  antecedent  streams.  It  is 
doubtful  if  there  are  many  cases  of  rivers  now  crossing  a 
large  mountain  in  exactly  the  same  course  which  they  occu- 
pied at  a  time  antecedent  to  the  mountain  formation ;  but 
many  geologists  believe  that  the  Green  River,  where  it 
crosses  the  Uinta  Mountains  of  Utah,  is  an  illustration  of 
this  type  of  stream. 

Ordinarily  the  folding  would  locally  transform  the  river  to 
a  lake,  and  as  the  dam  continued  to  grow,  the  lake  would 
gradually  become  deeper  and  more  extensive.  With  the 
formation  of  the  lake  the  erosive  power  of  the  stream 
decreases ;  for  when  it  flows  from  the  body  of  quiet  water, 
it  has  been  robbed  of  its  sediment  supply,  and  is  therefore 
unable  to  do  much  erosive  work.  If  the  mountain  growth 
is  rapid,  it  may  even  cause  a  stream  to  flow  in  a  direction 
opposite  to  the  course  which  it  originally  had  —  it  maybe 
diverted  or  even  inverted.     Where  the  rocks  in  the  middle 


RIVER   VALLEYS.  279 

course  of  a  river  are  rapidly  folded  during  mountain  growth, 
a  stream  may  even  be  separated  into  two  parts. 

With  the  growth  of  the  mountain,  since  the  river  slope  is 
increased,  new  tasks  are  set  before  the  streams.  Gorges  and 
waterfalls  are  caused,  and  because  of  the  great  elevation  of 
the  mountains,  these  continue  for  a  long  time  ;  and  thus  long- 
continued  youth  is  impressed  upon  the  mountain  valleys. 
Everv  mountain  furnishes  illustrations  of  these  latter  feat- 
ures ;  and  in  many,  such  as  the  Alps,  there  are  also  lakes, 
which  are  the  result  of  mountain  folding,  and  which  repre- 
sent the  interference  with  stream  erosion  which  is  brought 
about  by  the  growth  of  mountain  dams. 

Climatic  Accidents.  —  A  change  of  climate  to  a  condition 
of  dryness,  robs  streams  of  their  erosive  power  ;  but  even 
more  markedly  does  it  decrease  the  power  of  weathering. 
Hence  such  a  change  favors  the  angular  type  of  valley.  It 
reduces  the  number  of  streams  (Fig  150),  and  causes  those 
which  remain,  to  be  dry  for  a  large  part  of  the  time ;  and 
hence  in  a  dry  country,  there  are  large  areas  unoccupied  by 
drainage  lines.  A  rare,  heavy  rain,  falling  upon  such  an  un- 
drained  surface,  carves  a  temporary  valley,  or  arroi/a,  which 
may  never  again  be  occupied  by  water.  Stream  valleys  may 
be  permanently  abandoned,  while  others  may  be  only  withered 
or  shrunken.  By  the  increasing  dryness  of  the  climate,  lakes 
may  be  evaporated  and  great  basins  of  interior  drainage  be 
formed.  Therefore,  stream  systems  may  be  dissected  by  this 
cause  also,  and  channels  of  outflow  of  lakes  may  be  aban- 
doned (Fig.  Ie51),  while  the  direction  of  the  drainage  changes 
from  the  sea  to  the  lowest  point  of  the  old  lake  bottom  ;  and 
this  causes  many  other  peculiar  changes  of  a  minor  nature. 
By  this  action,  a  part  of  the  Great  Basin  which  was  once 
tributary  to  the  Pacific,  through  the  Columbia,  is  now  trans- 
formed to  the  Great  Salt  Lake  interior  drainage  area. 


280 


PHYSICAL  GEOGRAPHY. 


The  change  in  climate  which  produces  glaciation,  first 
covers  all  the  country  with  ice  and  buries  the  valleys.  Near 
the  margin  of  the  snow-covered  area,  streams  may  be  sepa- 
rated, and  an  entire  change   in   the    drainage    be    caused. 


Fig.  150. 
The  drainage  of  an  arid  region 

Among  the  effects  of  the  ice  front,  is  the  interference  with 
streams  that  flow  toward  the  ice,  which  acts  as  a  dam,  trans- 
forming them  to  lakes,  and  causing  them  to  overflow  across 
some  divide.     When  the  ice  of  the  North  American  conti- 


mVEU   VALLEYS. 


281 


^.^ 


■% 


nental  glacier  (see  Chapter  XVII.)  was  melting  from  the 
surface  of  the  country,  many  such  lakes  were  produced,  and 
some  of  them  were  of  great  size.  The  St.  Lawrence  system 
was  dammed,  and  lakes  were  produced  in  different  positions 
from  those  occupied  by  the  present  Great  Lakes.  During 
the  same  period,  the  valley  of 
the  Red  River  of  the  North 
was  transformed  to  a  great 
lake  which  overflowed  to  the 
Gulf  of  Mexico,  instead  of  to 
the  Arctic,  as  the  present 
drainage  directs. 

Some  streams  had  their 
courses  permanently  changed 
and  even  reversed.  When  the 
ice  melted,  it  left  much  drift 
material  upon  the  surface  ;  and 
this  soil  sometimes  completely 
buried  the  old  valleys,  so  that 
entirely  new  channels  had  to 
be  cut.  More  often  this  filling 
was  only  partial,  and  streams 
were  turned  from  their  course 
for  short  distances,  and  often 
dammed  into  lakes,  which  in 
many  cases  are  now  repre- 
sented by  swamps  (Plate  25).  Hence  in  a  glaciated  region 
we  may  have  very  complex  streams;  for  in  broad,  mature 
valleys,  local  post-glacial  gorges  may  be  cut,  while  here  and 
there  falls  and  lakes  exist.  The  streams  are  often  given 
new  life,  or  rejuvenated,  either  through  their  entire  course, 
or  for  a  short  distance.  Often  the  course  forced  upon  them 
is  very  much  more  roundabout  than  that  pursued  before  the 


.sSkSv^C 


U 


Fig.  151. 
The  Great  Basin.    The  lighter  shad- 
ing shows  the  former  extension  of 
lakes  when  the  Great  Salt  Lake 
overflowed  into  the  Columbia. 


282 


PHYSICAL   GEOGRAPHY. 


glacial  period  (Fig.  152).     Illustrations  of  the  various  effects 
of  glaciation   abound   by  the  thousand   in  the  glacier  belt 

of  New  England, 
New  York,  and 
other  of  the 
Northern  States. 
Nearly  all  of  the 
gorges  and  lakes 
in  this  belt  are 
the  result  of  the 
condition  of  glaci- 
ation. While  the 
most  notable  in- 
stances are  those 
of  the  Great 
Lakes     and     Ni- 


FiG.  152. 

Diagram  of  a  river  caused  to  flow  irregularly  because 
of  glacial  deposits  in  its  course,  which  prevented  it 
from  entering  the  main  stream  by  its  preglacial 
course,  now  partly  occupied  by  a  tiny  stream. 


agar  a,  these  are  merely  large  examples  of  a  great  group. 

Other  Accidents. — Interference  with  river  valley  develop- 
ment is  commonly  noticed  in  regions  of  volcanic  eruptions. 
Sometimes  the  valleys  are  filled  with  lava  ;  at  times  the 
streams  are  forced  to  cut  new  valleys  in  a  part  of  their 
course  ;  again  they  are  transformed  to  lakes  ;  and  they  may 
even  be  forced  to  flow  in  a  reversed  direction.  Here  again, 
the  valleys  are  rejuvenated,  and  gorges  and  falls  are  pro- 
duced. Illustrations  of  these  features  may  be  seen  on  almost 
any  map  of  a  region  of  volcanic  activity. 

An  avalanche  in  a  mountain  may  produce  one  or  all  of 
these  effects,  and  there  are  other  minor  accidents  to  which 
streams  are  subjected.  Sometimes  river  valleys  are  again 
and  again  subjected  to  one  or  several  of  these  accidents,  and 
their  cycle  of  development  much  interfered  with.  This  is 
why  youth  and  early  maturity  are  the  characteristic  features 
of  most  valleys  ;  for  the  stage  of  old  age  cannot  be  reached. 


BIVEB   VALLEYS, 


283 


SCALE  OF  MILES 
' 


S.D.SvtosiJl^, 


■.i 


0     K      1  2 

Plate  25. 
Drainage  in  the  glaciated  region  of  Wisconsin,  showing  the  abundant 
swamps  (indicated  by  dashes)  between  the  drift  hills,  and  the  interfer- 
ence of  these  hills  with  the  stream  course. 


284  PHYSICAL   GEOGRAPHY. 

because  in  the  conflict  between  denudation  and  the  internal 
forces  of  elevation,  the  latter  are  more  powerful  and  keep 
the  streams  either  constantly  or  intermittently  at  work  in 
valley  formation. 


REFERENCE   BOOKS. 

From  the  text  books  of  geology,  previously  referred  to,  one  may  obtain 
additional  information  upon  some  parts  of  the  subject  treated  in.  this  chapter. 

Important  articles  on  the  Development  of  Rivers  will  be  found  in  the 
National  Geographic  Magazine,  Washington,  D.C.,  Volumes  I.  and  II.  These 
are  from  the  pen  of  Professor  W.  M.  Davis.  This  magazine  is  a  very  valu- 
able one  for  teachers  of  geography.  Six  volumes  have  been  published,  the 
price  to  the  public  being  ^2.00  each  for  the  first  two,  and  $3.00  for  the 
others.  To  members,  they  are  sold  at  a  lower  rate ;  and  each  member  receives 
the  Magazine.  The  cost  of  membership  is  $2.00  a  year,  and  any  one  inter- 
ested in  geography  is  eligible. 

The  remarkable  Colorado  Canon  is  fully  described  by  Dutton  in  Mono- 
graph II  (with  Atlas),  U.  S.  Geological  Survey,  Washington,  1885.  $10.00. 
The  Atlas  is  splendidly  illustrated.  For  a  shorter  account,  see  Second 
Annual  Report  U.  S.  Geological  Survey,^  Washington,  1882.  Powell's 
"Exploration  of  the  Colorado  River  of  the  West,"  Washington,  1872,  now 
unfortunately  out  of  print,  but  still  on  sale  at  the  second-hand  stores,  is  a 
most  fascinating  description  of  travel,  as  well  as  a  scientific  description  of 
this  wonderful  region.  The  same  author  has  published  upon  the  same 
subject  "Canyons  of  the  Colorado."  Flood  «&  Vincent,  Meadville,  Penn., 
1895.    4to.     $10.00. 

Huxley.  —  Physiography.  Macmillan  &  Co.,  New  York,  1891.  12mo.  $1.80. 
(A  study  in  physical  geography,  in  which  the  Thames  Basin  is  taken  as 
the  central  topic.) 

1  Many  of  the  annual  reports  of  this  survey  may  be  obtained  by  the  aid  of  congressmen, 
though  the  earlier  ones  are  now  exhausted.  They  contain  much  valuable  material,  written  in  a 
sufficiently  popular  manner  for  the  non-geological  reader.  Eeference  is  made  to  many  of  these 
articles  in  the  later  chapters. 


CHAPTER  XVI. 

DELTAS,   FLOODPLAINS,    WATERFALLS,    AND   LAKES. 

Deltas.  —  Nearly  all  streams  carry  sediment ;  and  if  for  any 
reason  the  velocity  is  suddenly  checked,  some  of  this  material 
must  be  deposited.  The  most  favorable  situation  for  the 
deposit  of  river  sediment,  is  where  the  stream  enters  another 
body  of  water.  In  such  places  the  material  is  deposited 
near  the  stream  mouth,  and  a  delta  often  results. 

Where  streams  come  from  steep  mountain  valleys  upon 
relatively  level  plateaus,  the  sudden  change  in  slope  causes 
the  deposit  of  some  of  the  sediment  at  the  mountain  base. 
This  material  is  dropped  most  abundantly  near  the  moun- 
tain, and  the  rapidity  of  deposit  decreases  away  from  it. 
As  a  result  of  this,  a  fan- shaped  deposit  is  produced,  to 
which  the  names  alluvial  fan^  fan  delta^  or  cone  delta  are 
commonly  given  (Fig.  155).  These  deposits  are  very  com- 
mon in  arid  regions  ;  and  although  relatively  rare  elsewhere, 
when  they  occur  in  moist  countries,  they  are  usually  flatter 
and  less  distinct.  The  apex  of  the  fan  extends  up  the  stream 
toward  the  mountain  base. 

The  formation  of  true  lake  or  ocean  deltas,  depends  upon 
a  variety  of  circumstances.  There  are  many  large  streams 
which  are  not  forming  deltas  in  the  sea.  In  some  cases  this 
is  due  to  the  fact  that  the  streams  carry  very  little  sediment; 
in  other  cases,  the  sediment  brought  to  the  sea  is  mostly  car- 
ried away  by  currents.  In  general,  delta  formation  is  not 
favored  in  open  seas,  where  tidal  currents  and  waves  are 

285 


286 


PHYSICAL   GEOGRAPHY. 


present  to  distribute  tlie  sediment  over  the  ocean  bottom. 
Nearly  tideless  seas,  such  as  the  Gulf  of  Mexico  (Fig.  153), 
or  the  Mediterranean,  are  particularly  liable  to  have  deltas 
opposite  the  stream  mouths. 


Fig.  153. 

Delta  of  the  Mississippi. 

If  the  ocean  bottom  is  sinking,  the  rate  of  deposit  of  mate- 
rial opposite  the  river  mouth  is  often  not  sufficiently  rapid 
to  build  a  delta  above  the  level  of  the  sea  ;  and  therefore  for 
the  rapid  development  of  deltas,  the  ocean  bottom  must  either 
remain  in  its  position,  or  else  be  slowly  rising.  On  many 
seacoasts,  one  or  the  other  of  the  conditions  which  favor 


DELTAS,   FLOODPLAINS,  WATERFALLS,   ETC.         287 

delta  formation  is  absent,  and  this  is  the  reason  why  deltas 
in  the  sea  are  so  uncommon.  In  many  cases  the  submer- 
gence of  the  coast  has  transformed  the  river  mouths  to 
estuaries,  instead  of  admitting  t)f  the  formation  of  deltas. 

By  far  the  most  favorable  conditions  for  the  formation  of 
deltas  are  found  in  lakes.  Here  there  are  no  tides,  waves 
are  only  moderate  in  effectiveness,  and  the  depth  is  compara- 
tively shallow  and  usually  not  increased  by  subsidence  of  the 
bottom.  The  lake  water  acts  as  a  filter,  removing  all  the 
sediment  which  streams  bring,  and  the  greater  part  of  this 
is  deposited,  almost  immediately  opposite  the  mouth  of  the 
tributary.  With  these  very  favorable  conditions,  in  nearly 
every  lake  deltas  occur  opposite  the  mouths  of  most  of  the 
streams  ;  and  in  some  cases,  by  the  growth  of  two  deltas 
from  opposite  sides,  lakes  are  divided  into  two  parts,  as  at 
Interlaken  in  Switzerland. 

Over  large  deltas,  the  streams  flow  in  uncertain  course, 
sometimes  changing  their  channel  from  one  side  of  the  delta 
to  the  other,  as  is  so  frequently  done  on  the  delta  of  the 
Yellow  River  of  China.  In  this  way  much  destruction  of 
life  is  accomplished.  Over  the  nearly  level  delta,  the  main 
stream  divides  and  often  subdivides,  entering  the  sea  through 
a  number  of  branches,  which  may  be  called  distributaries, 
in  distinction  from  the  tributaries,  which  bri?i^  water  to  the 
stream,  while  these  distribute  the  river  water  to  the  sea 
(Fig.  153).  As  a  result  of  this  branching  of  the  streams, 
and  the  changes  in  river  channel,  in  course  of  time  all  parts 
of  the  delta  are  traversed  by  sediment-bringing  water ;  and 
in  this  way  the  delta  front  is  made  to  advance  into  the  sea, 
while  the  delta  itself  is  built  up  above  the  sea  level  (Fig. 
154).  In  the  course  of  the  growth  of  the  delta,  the  advance 
is  often  irregular,  and  arms  of  the  sea  may  be  enclosed  in 
the  form  of  lakes  (Fig.  153).     The  form  of  the  delta  is 


288 


PHYSICAL   GEOGRAPHY, 


roughly   triangular,    or   like   the    Greek   letter   Delta   (A), 
whence  its  name.     This  is  really  a  partial  though  somewhat 
distorted  cone,  not  unlike  the  fan  delta  itself  (Fig.  155). 
Floodplains.  —  Rivers  are  very  often  given  more  load  than 

they  are  able  to  carry, 
and  of  necessity  they 
are  obliged  to  deposit 
some.  The  material  is 
sometimes  deposited  in 
the  form  of  bars  in  the 
stream  channel,  or  at 
other  times  it  is  spread  over  the  valley  at  one  side  of  the 
channel,  particularly  when  the  stream  has  quantities  of  sedi- 
ment during  flood  times.  In  this  way,  by  laying  aside  parts 
of   the   sediment   load,  the    stream   is  forming  floodplains. 


Fig.  154. 
Diagram  to  show  the  mode  of  formation  of  a 

delta. 


Fig.  155. 
All  alluvial  fan. 


There  are  numerous  ways  in  which  these  may  be  caused. 
They  are  sometimes  merely  temporary  deposits,  being  formed 
at  the  same  time  that  the  stream  is  cutting  its  channel  deeper. 
At  certain  seasons  of  the  year,  the  river  is  obliged  for  a  while, 


DELTAS^   FLOODPLAINS,  WATERFALLS,    ETC.         289 

and  locally,  to  put  aside  some  of  its  load,  and  this  it  does, 
forming  narrow  floodplains  which  are  often  composed  of  very 
coarse  materials  (Fig.  156).  We  find  such  floodplains 
very  commonly  among  the  mountain  streams. 

Usually  floodplains  are  due  to  a  decrease  in  the  river  slope, 
a  decrease  which  normally  occurs  between  the  headwaters 
and  the  mouth.     Supplied  with   much   material   from   the 


Fig.  156. 
River  bed  and  floodplain  among  the  mountains. 

upper  parts  of  the  valley,  the  stream  reaches  these  regions  of 
less  slope  with  decreased  ability  to  transport  the  sediment ; 
and  some  of  it  must  be  deposited.  This  is  due  to  the  fact 
that  streams  are  able  to  transport  sediment  in  proportion  to 
their  velocity,  which  itself  depends  partly  upon  slope  and 
partly  upon  volume.  By  far  the  greater  number  of  the 
large  floodplains  of  the  world  are  due  to  this  decrease  in 
river  slope,  from  upper  to  lower  portions, 
u 


290  PHYSICAL   GEOGRAPHY. 

Sometimes  the  broad  floodplain  is  in  part  a  delta,  which 
has  been  left  inland  by  the  encroachment  of  the  delta  upon 
the  sea.  In  the  Mississippi  valley,  the  delta  began  to  form 
above  the  northern  limits  of  the  state  of  Mississippi,  and 
has  grown  outward  into  the  Gulf,  filling  the  estuary  which 
existed  there,  and  transforming  it  to  a  broad  floodplain,  as 
we  now  find  it.  This  change  is  something  like  that  which 
would  happen  if  the  streams  now  entering  Chesapeake  Bay 


I'lu.  157. 
Floodplain  of  a  great  river. 

should  fill  up  the  bay,  as  they  are  doing,  and  change  it  to  a 
level  plain  composed  of  fine-grained  materials  brought  down 
by  the  rivers. 

A  change  in  the  level  of  the  land,  tilting  the  seaward  por- 
tion of  a  stream  so  as  to  decrease  the  slope,  may  also  bring 
about  conditions  favoring  the  formation  of  floodplains ;  and 
any  cause  which  increases  the  sediment,  also  favors  this 
formation.  If  a  stream  channel  is  graded  to  a  given  volume 
of  water  and  sediment  load,  an  increase  in  the  sediment  will 
necessitate  the  deposit  of  some,  and  this  will  produce  a  flood- 
plain  ;  or  a  decrease  in  the  volume,  such  as  would  result  in 


DELTAS,   FLOODPLAINS,  WATERFALLS,   ETC. 


291 


the  change  of  climate  from  moist  to  dry,  if  the  sediment 
load  is  not  also  decreased,  will  bring  about  floodplain  forma- 
tion. From  this  it  is  seen  that  floodplains  are  formed  by 
quite  different  causes. 

Their  characteristics  are  rather  simple.  For  the  most  part 
they  consist  of  remarkably  level  plains  (Fig.  157),  usually 
partly  swampy,  and  composed  of  fine  soil,  which  is  generally 


Fig.  158. 

SO  rich  that  the  floodplain  regions  are  important  agricul- 
tural districts.  The  main  stream  meanders  through  the 
plain  in  great  swinging  curves  (Figs.  135, 172,  and  157-160), 
so  that  its  course  is  sometimes  greatly  increased  in  length. 
On  the  Mississippi,  a  steamer  is  often  within  a  few  hundred 
yards  of  a  portion  of  the  river  which  can  be  reached  by  water 
only  by  a  sail  of  several  miles.  These,  which  are  known  as 
oxbow  curves,  are  constantly  changing  in  form  and  hence 


292 


PHYSICAL   GEOGBAPST, 


Fig.  159. 


in  position.     The  river  is  eating  its  way  into  the  floodplain 
on  the  concave  bank,  and  depositing  npon  the  convex  bank 

(Figs.  158-160,  in  which 
the  dotted  areas  repre- 
sent sand  deposits) .  This 
process  of  change  often 
causes  the  river  to  cut 
across  the  narrow  neck  of 
land  between  two  parts 
of  the  curve,  and  thus 
shorten  the  course  and 
abandon  the  old  curve. 
In  delta  and  floodplain 
regions,  these  are  known 
as  oxbow  cut-offs  (Fig. 
159)  ;  and  after  they  are 
formed,  they  become  crescent-shaped  lakes,  and  sometimes 
they  are  almost  complete  circles.  In  the  course  of  time  these 
lakes  are  destroyed 
by  being  filled  with 
sediment  when  the 
stream  is  in  flood, 
and  when  the  flood- 
plain  is  submerged 
beneath  the  river 
water  (Fig.  160). 
These  great 
floodplains  are  con- 
stantly  being 
raised  by  the  de- 
posit of  sediment ; 
and  the  time  of  their  formation  is  that  of  the  flood  stage  of 
the  stream,  wdien  it  is  no  longer  confined  to  its  channel,  but 


Fig.  160. 


DELTAS,   FLOODPLAINS,  WATERFALLS,   ETC.         293 

overflows  and  submerges  the  great  level  tracts  on  either  side. 
Sediment  is  being  deposited  from  this  great  expanse  of  water, 
because  the  velocity  is  decreased  in  these  shallow  areas.  It 
is  to  prevent  this  flooding  that  the  levee  banks  are  built  on 
the  margin  of  the  floodplain  of  the  Mississippi.  These  banks 
are  built  to  a  sufficient  height  to  shut  out  the  high  water 
from  the  flood- 
plains. 

-  While  the  stream 
is  constantly  at 
work  building  up 
its  floodplain  dur- 
ing floods,  by  its 
meandering  it  is 
constantly  at  work 
removing  por- 
tions; and  so  there 
is  a  process  of  in- 
termittent move- 
ment of  sediment, 
from  up  stream 
down  toward  the 

mouth.     It  is  de- 

•  i.     J    J        •  Fig.  161. 

posited  during 

n       11,       . ,  Falls  of  the  Yellowsk)ne. 

nood  ;  later  it  may 

be  attacked  by  the  lateral  cutting  of  the  stream ;  and  then 
it  is  carried  a  step  down  stream,  perhaps  to  be  deposited 
again,  and  then  after  awhile  to  again  start  in  movement. 

Upon  a  floodplain  the  tributaries  to  a  river  enter  the 
main  stream  at  very  acute  angles.  The  slope  is  so  gentle, 
that  the  deposit  of  sediment  near  the  mouth  of  the  tributary 
constantly  tends  to  divert  the  river  further  and  further  down 
stream.     On  floodplains,  the  tributaries  often  flow  for  many 


294 


PHYSICAL   GEOGRAPHY. 


miles  in  a  course  nearly  parallel  to  the  stream  which  they 
would  join ;  and  in  some  rivers,  the  tributary  streams  have 
been  so  far  deflected  that  they  enter  the  sea  independently. 

Waterfalls.  —  When  for  any  reason  a  stream  has  a  sudden 
descent  in  its  channel,  waterfalls  or  rapids  are  produced 
(Figs.  161-166)  ;  and  we  cannot  separate  the  two  phenom- 
ena, because  there  is  every  gradation  between  them.  There 
are  many  ways  in  which  an  unnaturally  steep  slope  may  be 
introduced  into  the  stream  channel.  One  of  the  most  com- 
mon means  is  by  the   accidental   diversion   of  the   stream 

from  its  course.  The  great 
majority  of  waterfalls  in  the 
United  States  have  been 
caused  hy  changes  in  the 
stream  courses,  the  result 
of  some  interference  on  the 
part  of  glacial  deposits.  As 
a  result  of  these  glacial  drift 
accumulations  in  stream 
valleys,  in  many  cases  the 
rivers  have  been  turned  to 
one  side,  and  caused  to  flow 
over  steep  descents,  produc- 
ing either  a  series  of  rapids 
or  of  waterfalls  (Fig.  162). 
The  thousands  of  waterfalls 
in  northern  United  States 
are  mostly  the  direct  result 
of  this  kind  of  accident ;  and 
Niagara  (Figs.  132  and  163)  may  be  taken  as  a  typical  illus- 
tration of  this  kind  of  waterfall. 

At  the  close  of  the  glacial  epoch,  the  Niagara  River  flowed 
from  Lake  Erie  to  Ontario  along  its  present  course,  and 


Fig.  162. 

Taughannock  Falls,  New  York.  Caused 
by  change  in  a  river  course  due  to 
glacial  obstructions. 


DELTAS,   FLOOBPLAINS,  WATERFALLS,   ETC.         295 

entered  Ontario  after  a  sudden  descent  over  the  bluffs  at 
Queenstown  (Fig.  169).  Glacial  deposits  left  by  the  ice  had 
so  filled  the  old  channel,  that  this  new  course  was  the  natural 
outflow  of  Lake  Erie.  The  waterfall  produced  in  this  way, 
has  been  gradually  retreating  backward  toward  Lake  Erie, 
and  at  present  is  seven  miles  from  its  former  position.     In 


Fig.  163. 
American  Falls,  Niagara. 

the  process  of  this  retreat,  the  gorge  has  been  cut  to  a  depth 
of  from  200  to  300  feet,  with  a  width  of  from  200  to  400  yards, 
while  the  fall  itself  is  now  about  160  feet  in  height.  Careful 
surveys  made  many  years  apart,  show  that  the  retreat  of  the 
waterfall  toward  Lake  Erie  is  rather  rapid,  on  the  aver- 
age being  not  far  from  five  feet  a  year.  If  this  average 
has  been  maintained  throughout  the  entire  history  of  Niag- 


296 


PHYSICAL   GEOGRAPHY. 


ara,  the  time  occupied  in  cutting  the  gorge  from  Queenstown 
to  the  base  of  the  falls,  is  somewhere  between  7000  and 
10,000  years.  The  falls  of  St.  Anthony,  in  the  Mississippi 
valley,  are  of  the  same  origin,  and  have  had  nearly  the 
same  history ;  and  the  same  is  true  of  a  vast  number  of 
waterfalls  in  the  northern  states  of  the  Union. 

Any  other  obstacle  in  the  way  of  a  stream  will  transform 
it  into  a  waterfall,  such  for  instance  as  the  folding  of  moun- 


Fig.  164.    Yosemite  Falls. 

tains,  or  the  passage  of  a  lava  flow  across  a  stream  valley,  or 
any  one  of  several  similar  accidents.  When  rocks  break, 
and  move  on  one  side  of  the  crack,  as  is  done  when  faults 
occur,  the  movement  increases  the  slope  of  the  stream  near 
the  fault  line.  Thus  between  the  plains  bordering  the 
eastern  coast  of  the  United  States,  and  the  hilly  region  just 
inland  from  these,  there  is  a  line  of  movement,  on  the  land- 
ward side  of  which  the  country  has  been  raised;  and  this 
line  has  determined  the  existence  of  a  large  number  of  small 
falls  and  rapids.     Because  of  this  it  has  been  called  the  fall 


DELTAS,   FLOODPLAINS,   WATERFALLS,  ETC.  297 


line;  and  this  small  geological  accident  has  been  largely 
responsible  for  the  location  of  several  of  the  great  cities  along 
the  Atlantic  coast.  The  falls  and  rapids  mark  the  approxi- 
mate limit  of  navigable  waters,  for  ships  cannot  pass  over 
them;  and  since  the  cities  were  so  placed  in  order  that  they 
might  have  the  advantage  of  ocean  traffic,  and  still  be  as  far 
inland  as  possible,  they 
were  usually  located  at 
the  head  of  navigation. 
Thus  such  cities  as  Phila- 
delphia, Baltimore,  Wash- 
ington, and  others,  are 
situated  just  on  the  sea- 
ward side  of  this  fall  line, 
and  small  falls  and  rapids 
are  found  almost  within 
the  city  limits. 

As  a  stream  deepens 
its  cliannel,  it  may  actu- 
ally form  waterfalls  as  a 
result  of  its  work.  The 
river  is  able  to  remove 
soft  rocks  more  rapidly 
than  hard  ones,  and  if 
the  stream  channel  is 
crossed  by  layers  of  dif- 
ferent hardness,  the  differ- 
ence in  rate  of  cutting  in 
the  two  kinds  of  rock  will  produce  a  rapid,  or  even  a  water- 
fall (Figs.  162, 163,  and  165).  The  hard  layer  tends  to  stand 
up  above  the  soft  one,  and  thus  there  is  a  steep  descent  in  the 
stream  valley.  As  soon  as  the  stream  has  cut  doAvn  to  the  line 
where  its  power  of  deepening  ceases,  the  waterfalls  disap- 


FiG.  165. 

Small  waterfalls  in  a  gorge  near  Ithaca, 
N.Y.,  where  the  water  flows  over  nearly 
horizontal  rocks  of  varying  hardness. 


298 


PHYSICAL   GEOGBAPHT. 


^„r 


pear.  Falls  of  this  origin  are  particularly  common  in  regions 
of  horizontal  rocks ;  for  here  the  waterfall  tends  to  retreat 
upstream  (Fig.  166),  and  hence  remains  for  a  long  time.  In- 
deed, it  remains  until  the  stream  has  eaten  its  way  far  enough 
back  to  have  escaped  these  differences  in  rock  structure. 
There   are  other   causes   for   waterfalls   and   rapids,   but 

none  of  especial  importance.  Perhaps 
one  of  these  kinds  should  be  men- 
tioned ;  that  is  the  one  so  well  illus- 
trated in  the  valley  of  the  Colorado 
River  of  the  West.  During  times  of 
heavy  rains,  the  streams  tributary  to 
this  river  bring  to  the  main  stream 
vast  quantities  of  material,  sometimes 
boulders  weighing  tons.  They  are 
able  to  do  this  because  they  enter 
the  main  stream  with  very  rapid 
slope,  —  much  more  rapid  than  that 
of  the  Colorado  itself.  Opposite  their 
mouths  they  build  up  these  coarse 
fragments,  which  the  river  itself  is 
not  able  to  remove  ;  and  over  these  bars  the  water  flows 
in  rapids,  which  are  sometimes  so  well  developed  that 
it  is  almost  impossible  to  travel  down  the  stream  in  a  boat. 
Only  one  or  two  parties  have  succeeded  in  passing  through 
this  canon,  and  they  experienced  many  dangers  which  were 
caused  by  the  rapids  of  this  origin. 

Lakes.  ^- A  lake  is  properly  a  part  of  a  river,  and  it  may 
have  been  formed  by  one  of  several  causes.  There  are  many 
differences  in  lakes ;  some  are  fresh,  others  salt ;  some  have 
tributaries  from  the  surface,  others  are  mainly  if  not  entirely 
supplied  with  water  from  underground ;  some  have  outlets, 
and  others  are  without  them.     In  form  and  in  depth  there  is 


Fig.  166. 

To  illustrate  the  probable 
condition  at  Niagara, 
where  the  water  falls 
over  a  hard  limestone 
stratum,  beneath  which 
are  softer  layers. 


DELTAS,   FLOODPLAINS,  WATERFALLS,   ETC. 


299 


almost  infinite  variety;  but  in  all  cases  they  will  be  found  to 
be  parts  of  river  systems. 

Anything  that  changes  a  stream  valley  so  that  the  bottom 
becomes  a  trough  or  basin,  will  produce  a  lake.  By  far  the 
most  common  cause  for  this  is  the  effect  of  glacial  deposits 
(Chapter  XVII.).  The  stream  valleys  which  were  carved 
before  the  ice  covered  the  country,  were  dammed,  or  in  other 
ways  interfered  with  by  glacial  deposits,  or  by  glacial  action, 
so  that  when  the  ice  retreated,  the  rivers  found  it  impos- 
sible to  flow  over 
the  land  without 
becoming  locally 
transformed  to 
lakes  (Fig.  167). 
The  scores  of 
thousands  of  lakes 
and  ponds  that 
exist  in  northern 
United  States  and 
Europe,  are  mostly 
due  to  glacial  ac- 
tion (Figs.  168  and 
190).  Other  acci- 
dents to  rivers 
may  produce  lakes  in  a  similar  way.  Thus  a  lava  flow 
may  dam  a  stream  and  form  a  lake ;  or  an  avalanche  may 
do  the  same ;  or  the  growth  of  a  mountain  across  a  stream 
valley  may  transform  it  to  a  body  of  quiet  water.  A  large 
majority  of  the  lakes  in  the  world  are  a  result  of  accidents, 
either  of  these  or  other  kinds.  In  many  cases  the  origin  is 
complex,  several  causes  uniting  to  produce  the  lake  basin. 

Not  a  few  lakes  in  the  world  are  the  result  of  other  causes. 
Original  depressions   on  the   surface   of  a  land  which   has 


Fig.  167. 
Avalanche  Lake,  Adirondacks,  N.Y.    Part  of  a  river 
valley  transformed  to  a  lake.     (Copyrighted,  1889, 
by  S.  R.  Stoddard,  Glens  Falls,  N.Y.) 


300 


PHYSICAL   GEOGRAPHY. 


been  newly  added  to  the  continent,  when  filled  with,  water 
are  formed  into  lakes.  This  is  the  origin  of  the  large  num- 
ber of  lakes  in  Florida,  and  of  Lake  Drummond  in  the  Dis- 
mal Swamp.  Others  may  be  produced  during  and  as  a 
result  of  the  natural  development  of  streams.  Such  lakes 
as  the  oxbow  cut-offs  described  above  (Figs.  135  and  160), 
or  those  formed  by  the  irregular  growth  of  deltas  (Fig.  153), 
are  dependent  upon  the  development  of  streams. 


Fig.  168. 
Glacial  lakes  in  the  Adirondacks. 


Lakes  are  merely  temporary  phenomena,  forming  but  one 
stage  in  river  development.  They  are  speedily  removed,  and 
any  lake  which  exists  in  the  course  of  a  stream,  acts  as  a  bar- 
rier to  river  development  so  long  as  it  remains.  The  removal 
of  lakes  is  usually  accomplished  by  the  combination  of  two 
processes  of  river  work  :  one,  the  filling  of  the  lake,  the  other, 
the  cutting  of  the  barrier.  Lake-filling  is  by  far  the  most 
important,  and  nearly  every  particle  of  sediment  that  comes 
into  the  lake  waters,  works  towards  this  end  of  destruction. 


DELTAS,   FLOODPLAINS,  WATERFALLS,   ETC, 


301 


Unless  the  conditions  are  exceptional,  the  process  of 
down-cutting  at  the  outlet  of  the  lake  is  relatively  unim- 
portant. For  streams  which  emerge  from  these  quiet  bodies 
of  water  have  very  little  working  power,  because  all  sedi- 
ment has  been  removed  by  the  lake,  and  the  stream  has 
thus  been  robbed  of  the  tools  with  which  it  commonly  does 
its  work.  It  is  still  able  to  act  chemically;  but  this  is  one  of 
the  least  important  means  which  streams  have  for  cutting 


'<7  .•*-"''iP^;<.'--^--^-«^':.--''~'--;^l&*    "**'5k£*- '*^-^." 


Fig.  1G9. 
Bird's-eye  view  of  Niagara  gorge  and  falls. 


their  channels.  For  instance,  the  Niagara  emerges  from 
Lake  Erie  through  a  valley  which  is  scarcely  perceptible 
(Figs.  132  and  169),  the  river  flowing  almost  on  the  sur- 
face of  the  plain;  and  in  all  the  time  that  this  stream 
has  drained  Lake  Erie,  it  has  done  almost  no  work  of 
channel  formation  between  the  lake  and  the  falls. 

If  the  rock  which  forms  the  barrier  to  a  lake  is  composed 
of  very  soft  materials,  which  the  water  is  easily  able  to 
remove,  or  if  it  is  easily  soluble,  the  barrier  may  be  rapidly 


302 


PHYSICAL   GEOGRAPHY. 


cut  down,  and  thus  the  lake  be  speedily  drained.  Or  if, 
after  emerging  from  the  lake,  the  stream  finds  itself  precipi- 
tated over  some  steep  slope,  its  power  of  working  is  so  con- 
centrated by  this  waterfall,  that  it  rapidly  wears  a  channel, 
as  has  been  done  by  Niagara  between  Queenstown  and  the 
falls.  Niagara  is  wearing  back  its  falls  towards  Lake  Erie ; 
and  given  time,  as  a  result  of  this  concentration  of  work,  it 
will  so  lower  the  outlet  as  to  completely  drain  Lake  Erie. 

Lakes  may  be  partially  or  entirely  destroyed  by  evapora- 
tion, as  has  been  the  case  in  the  great  interior  basin  of  the 


Fig.  170. 
Shore  lines  of  extinct  Lake  Bonneville. 

west.  Here  there  formerly  existed  numerous  large  lakes, 
some  of  which  had  outlets  to  the  sea  (Fig.  151).  By  a 
change  in  climate,  arid  conditions  replaced  those  of  moist- 
ness,  and  the  lakes  shrunk,  until  now  there  exist  in  their 
place,  either  alkaline  desert  plains,  or  shallow  salt  lakes 
without  outlet.  The  streams  are  constantly  bringing  small 
quantities  of  salt,  and  this  is  gradually  accumulated  as  the 
water  evaporates;  so  that  in  time,  the  fresh  water  becomes 
salt,  and  this  may  go  on  until  some  of  the  salt  is  precipitated 
in  the  lake  bottom. 


DELTAS,   FLOODPLAINS,  WATERFALLS,   ETC.         303 

A  change  to  a  moist  climate  would  again  transform  these 
basins  to  large,  fresh- water  lakes ;  and  in  the  complex 
history  of  that  interior  basin  region,  such  an  alternation 
of  climate  has  occurred.  The  geological  history  reveals 
two  moist  periods,  with  intervening  dryness;  and  now 
within  sight  of  Salt  Lake  City,  the  beaches  (Fig.  170), 
bars  and  cliffs,  formed  by  the  waters  of  these  ancient 
lakes,  may  be  readily  seen  extending  along  the  mountain 
base.      So  distinct  are  they,  that   even   the  cowboys  have 


Fig.  171. 
A  Florida  swamp. 


recognized  the  fact  that  water  formed  them.  One  of  these 
extinct  lakes,  the  ancestor  of  Great  Salt  Lake  (called  Lake 
Bonneville),  had  an  area  of  19,750  square  miles,  with  a 
depth  of  1050  feet.  It  covered  an  area  now  occupied  by 
fully  200,000  people,  and  its  depth  near  the  great  Mormon 
temple  was  850  feet. 

Swamps.  —  The  usual  way  in  which  lakes  are  removed,  is 
by  the  combination  of  the  two  processes  of  filling  and  down- 
cutting;    and  generally  lake-filling  is  Qf   more  importance 


304 


PHYSICAL   GEOGRAPHY. 


r  ii: 


liZ. 


Ray  Brook,  Adirondacks. 


than  the  down-cutting  of  the  outlet.  In  the  glacial  belt  of 
northern  United  States,  where  lakes  of  all  sizes  were  formed 
when  the  ice  retreated,  we  find  abundant  illustration  of  every 
stage  in  the  destruction  of  lakes.  The  more  shallow  of  these 
have  been  transformed  to  swamps,  which  are  usually  a  final 

stage  in  the  process  of 
lake  destruction  (Fig. 
172).  After  the  sedi- 
ment  has  elevated  the  bot- 
tom of  the  lake  nearly  to 
the  surface  of  the  water, 
vegetation  commences  to 
grow  and  to  increase  the 
rapidity  of  lake-filling. 
At  first  the  plants  are 
sedges  and  other  species  characteristic  of  lakes  ;  then  they 
are  replaced  by  mosses  ;  and  finally  the  swamp  becomes 
transformed  to  a  forested  area,  which  is  the  last  step  in  the 
change  from  lake  to  dry  land. 

There  are  other  causes  for  fresh-water  swamps.  The 
interference  with  drainage  on  the  part  of  vegetation,  may 
produce  SAvamp  conditions.  The  sphagnum  moss,  which  is 
the  form  of  vegetation  causing  the  peat  bogs  of  the  north, 
by  growing  near  the  outlet  of  springs  may  transform  these 
into  bog  areas,  even  upon  hillsides ;  and  the  growth  of 
reeds,  and  other  forms  of  vegetation,  along  sluggishly  mov- 
ing bodies  of  water,  may  transform  them  into  swamp  areas. 
The  Dismal  Swamp,  with  an  area  of  1500  square  miles,  ap- 
pears to  be  partly  due  to  this  cause.  The  flooding  of  rivers 
also  produces  swamp  conditions.  But  by  far  the  largest 
number  of  swamps  are  the  direct  result  of  the  destruction 
of  lakes.  This  is  illustrated  in  the  Florida  swamps  (Fig. 
171),  as  well  as  in  those  of  the  glacial  belt, 


BELT  AS,   FLOODPLAINS,  WATERFALLS,    ETC.         305 


REFERENCE   BOOKS.i 

The  best  treatise  upon  lakes  known  to  the  author  is  Gilbert's  Lake 
Bonneville,  Monograph  I.,  U.  S.  Geological  Survey,  Washington,  1890. 
$1.50.  (A  treatise  not  merely  on  this  one  lake,  but  upon  many  allied  sub- 
jects. An  abstract  of  this  appeared  in  the  Second  Annual  Report,  U.  S. 
Geological  Survey,  Washington,  1882.) 

See  also  Russell's  Lake  Lahontan,  Monograph  XL,  TJ.  S.  Geological 
Survey,  Washington,  1885.  $1.75.  (Short  abstract  of  the  same  in  tlie 
Third  Annual  Report  of  the  Geological  Survey,  1883.  In  later  reports 
there  are  one  or  two  other  articles,  by  the  same  author,  on  the  ancient  lakes 
of  the  Great  Basin.) 

For  Swamps,  see  Shaler,  U.  S.  Geological  Survey,  Tenth  Annual  Report, 
Washington,  1890. 

For  Niagara,  see  Gilbert's  discussion  in  the  Smithsonian  Annual  Report 
for  1890  (pages  231-257),  Washington,  D.C. 

For  Shore  lines,  see  references  for  Chapter  XVIII. 

1  The  subjects  of  this  chapter,  as  of  some  others,  are  not  yet  treated  in  a  complete  way  in 
books  of  popular  interest,  and  the  literature  is  widely  scattered,  and  often  in  very  inaccessible 
publications.  In  some  of  the  text  books,  and  general  books  of  reference,  these  subjects  are 
treated  from  certain  standpoints.  Some  of  the  monographs  of  the  National  Geographic 
Society  (published  for  use  in  the  schools,  at  the  price  of  $0.20  each)  now  being  issued  by  the 
American  Book  Co.,  New  York,  promise  to  fill  these  gaps.  Exact  reference  cannot  be  made 
to  them,  since  at  the  time  of  writing,  only  one  or  two  of  the  preliminary  numbers  have  been 
issued= 


CHAPTER   XYII. 


GLACIERS. 


Cause  of  Glaciers.  —  A  glacier  is  an  accumulation  of  snow, 
for  the  most  part  solidified  into  ice,  which  is  engaged  in 
a  slow  movement  from  one  place  to  another.  When  the 
snowfall  is  so  great  that  the  warmth  of  summer  is  unable 
to  entirely  re- 
move it,  the  con- 
ditions favoring 
the  formation  of 
a  glacier  are 
brought  about. 
Year  after  year 
the  snow  accu- 
mulates (Fig. 
173),  and  in  the 
course  of  time 
this  accumula- 
tion makes  move- 
ment necessary, 
for  it  flows  ac- 
cording to  cer- 
tain laws.  As  a 
result  of  this 
movement,  particularly  when  it  occurs  among  mountains, 
the  ice  stream  may  extend  far  below  the  snow  line  ;  and  in 
the  Alps,  the  ends  of  glaciers  are  sometimes  near  fields  of 
growing  grain.     They  extend  down  until  they  reach  a  place 

306 


Fig.  173. 
An  Alpine  snow  field. 


GLACIERS. 


307 


where  the  warmth  of  the  sun  is  sufficient  to  melt  them,  and 
therefore  to  stop  their  further  movement.  This  place  is  not 
a  fixed  line,  but  may  vary  from  year  to  year,  so  that  the 
front  of  a  glacier  often  retreats  and  advances. 

The   conditions   at   present    favoring    the    formation    of 
glaciers,  are  found  either  in  high  mountains,  or  else  in  lati- 


FiG.  174. 
Whitney  glacier,  Mt.  Shasta. 

tudes  within  the  Arctic  or  Antarctic  circles.  There  was  a 
time  when  these  conditions  existed  further  south,  and  then 
general  glaciation  was  brought  about  in  regions  now  within 
the  temperate  zone.  There  are  two  quite  distinct  classes  of 
glaciers :  the  valley  or  alpine,  and  the  continental  glacier. 

Alpine  or  Valley  Glacier.  —  This  form  of  glacier  receives 
its  name  from  the  fact  that  it  is  generally  developed  in 


308 


PHYSICAL   GEOGRAPHY. 


mountain  valleys,  and  is  particularly  well  developed  among 
the  Alps  (Fig.  175).  We  also  find  valley  glaciers  among 
most  of  the  mountains  of  Alaska  (Plate  26),  in  British 
Columbia,  in  some  of  the  high  mountains  of  Washington, 
such  as  Mt.  Shasta  (Fig.  174),  and  in  several  places  in 
the  Sierra  Nevadas  (Fig.  177).  The  glaciers  of  the  west 
are  small  and  insignificant,  but  those  of  Alaska  are  among 

the  best  developed 
in  the  world.  Val- 
ley glaciers  are  by 
no  means  uncom- 
mon in  other  parts 
of  the  earth  ;  and, 
among  other 
places,  we  find 
them  in  Norway, 
New  Zealand,  and 
Tierra  del  Fuego. 
In  most  of  the  al- 
pine glaciers  of 
the  northern  hem- 
isphere, there  is 
evidence  that  in 
the  period  imme- 
diately preceding 
the  present,  they 
extended  farther  down  their  valleys  than  at  present. 

The  valley  glacier  has  its  beginning  in  the  snow  field  of 
the  higher  portions  of  the  mountains,  which  are  the  great 
feeding  grounds  (Fig.  67).  Here  the  more  level  portions 
of  the  ground  are  permanently  covered  with  snow,  the 
accumulation  of  many  winters.  As  this  increases  in  depth, 
it  is  unable  to  remain  on  the  steeper  portions  and  drops 


Fig.  175. 

The  Rhone  glacier,  showing  the  ice  stream   from 
snow  field  to  terminus. 


GLACIERS. 


309 


down  the  hillsides  into  the  valleys,  in  the  form  of  great 
snow  avalanches.  Here  it  begins  a  slow  movement  down 
the  valleys,  whose  slopes  are  usually  steep;  and  in  the 
course  of  this  movement,  the  snow  becomes  compacted 
into  ice,  and  is  transformed  to  the  true  moving  glacier 
(Fig.  175).  The  rate  of  movement  is  exceedingly  slow, 
and  unless  watched  very  carefully,  is  not  noticeable.  In  a 
measure,  its  movement  may  be  compared  to  that  of  river  water, 
although  this  comparison  is  capable  of  being  extended  only 
in  a  very  general  way.  It  moves  more  rapidly  in  the  central 
portion  than  on  the  margins,  and,  like  water,  it  gradually 

moves  down  the  grades.      If    _ . 

the  valley  grade  is  regular,  the  t 
surface  of  the  ice  is  compara- 
tively smooth,  although  it  may 
here  and  there  be  creased  by 
fissures  or  crevasses  (Fig.  176). 
When  the  valley  bottom  is 
itself  very  irregular,  and  the 
slope  changeable,  the  ice  top 
may  become  transformed  to  a 
very  rough  surface,  which  is 
much  broken  and  difficult  to 
traverse,  and  which  may  be 
called  an  ice  fall.  By  melting, 
as  a  result  of  the  effect  of  the 
sun's  rays,  the  surface  of  the 
glacier  may  have  its  irregu- 
larities increased;  and  in  some 
cases  the  surface  of  a  valley  glacier  is  almost  impassable 
(Fig.  177). 

In  the  course   of   its   movements   down   the   valley,  the 
glacier  is  engaged  in  the  transportation  of  a  certain  amount 


Fig.  17(i. 
Crevasse  in  a  glacier. 


310 


PHYSICAL   GEOGRAPHY, 


of  rock  material.  Some  of  this  is  supplied  from  the  valley 
sides,  which  are  subjected  to  the  action  of  weathering,  and 
from  which  avalanches  are  not  uncommon.  As  a  result  of 
this,  the  margin  of  the  valley  glacier  is  usually  lined  with 
rock  fragments,  to  which  accumulation  the  name  lateral 
moraine  is  given.      Where   two  valley  glaciers   unite,  the 


Fig.  177. 
Glacier,  Mt.  Dana,  California,  showing  rough  surface  and  terminal  moraine. 

lateral  moraines  of  one  side  of  each  glacier  join  and  form  a 
moraine  in  the  center,  known  as  the  medial  moraine  (Plate 
26).  Some  of  this  rock  material  escapes  through  the  cracks 
to  the  bottom  of  the  ice,  and  this  is  dragged  along  the 
bottom,  giving  to  the  ice  a  power  which  on  a  large  scale  is 
not  unlike  that  of  sandpaper.  The  moving  ice  drags  these 
fragments  over  the  bottom,  and  scours  off  other  fragments 


l-H 

C 
I— ( 

s 

a 


t-i 

OS 
0) 

o 


o 

CO 

b» 

o 

CO 
CO 

c3 


O 

'3d 


312 


PHYSICAL   GEOGRAPHY, 


p■."AV0V:^VAC^v::"::•■Cr.■-/^^a■:■■::^^^0.^;^^/■^^^uv/:^::6:/:^■^u.■.^./.■/:O^.^ 


T 


Fig.  178. 

Section  of  a  glacier.    M,  medial ;  T,  terminal ;   and  G, 

ground  moraines. 


from  beneath.  This  material  also  is  carried  by  the  ice 
down  the  valley  in  the  form  of  a  ground  moraine  (Fig.  1T8). 
After  a  while,  the  glacier  comes   to   an    end  at  the  place 

where  the  melt- 
ing is  eqnal  to 
the  supply  of 
ice.  Here  much 
of  the  mate- 
rial that  was 
brought  on  the 
back  of  the  ice,  or  beneath  it,  is  deposited  at  the  frontal 
margin,  forming  a  terminal  moraine  (Figs.  177  and  178). 
The  melting  of  the  glacier  furnishes  water  for  a  stream, 
which  usually  emerges 
from  an  ice  cave  (Fig. 
179)  at  the  front  of  the 
glacier,  and  passes  down 
the  valley  as  a  muddy 
torrent,  carrying  with  it 
some  of  the  finer  parti- 
cles of  morainal  mate- 
rial. These  are  the  most 
characteristic  features  of 
the  valley  glacier. 

.  A  rather  peculiar  modi- 
fication of  valley  glaciers 
is  found  at  the  base  of 
the  Mt.  St.  Elias  group 
of  Alaska.  In  these 
mountains,  there  are 
many  large  and  beauti- 
fully developed  valley  glaciers  (Fig.  67),  which,  after  reaching 
the  foot  of  the  mountains,  extend  toward  the  sea  over  a  nearly 


Fig.  179. 
Ice  cave  at  terminus  of  a  glacier. 


GLACIERS.  313 

level  plain.    The  slope  of  the  plain  is  so  slight,  and  the  supply 

of  ice  so  limited,  that  this  part  of  the  united  glaciers  is  almost 

stagnant.     There  is  hardly  any  perceptible  movement;   and 

near  the  margin,  morainal  material  accumulates  on  the  surface 

of  the  ice  in  such  quantities  as  to  completely  bury  it,  forming 

a  soil  on  its  surface,  upon  which  vegetation  grows.    We  have 

on  this,  the  Malaspina  glacier,  an  instance  of  a  well-developed 

forest,  almost  as  luxuriant 

as  some  of  those  found  in 

the    temperate     latitudes, 

but  yet  growing  upon  the 

back  of   a  slowly  moving 

glacier.        A    forest    also 

extends  up    to    the    very 

base  of   the  glacier  (Fig. 

180).     This    form    of   an  ^.^^  -^^^ 

ice    sheet    has    been   called       Forest  at  the  margin  of  Malaspina  glacier, 

a  piedmont  glacier,  because  Alaska, 

it  is  developed  at  the  foot  of  mountains. 

Continental  Glaciers. — In  the  arctic  and  antarctic  zones, 
the  long  winter,  and  the  coolness  of  the  summer,  conspire 
to  bring  about  extensive  accumulations  of  snow  and  ice. 
As  a  result  of  this,  some  of  the  lands  in  these  cold  regions 
are  covered  with  great  sheets  of  ice ;  and  these  are  generally 
in  movement,  from  the  central  portion  of  the  land  mass, 
toward  the  sea.  In  Greenland,  and  on  the  Antarctic  land, 
they  are  so  large  as  to  warrant  the  name  continental,  for 
they  bury  lands  of  continental  extent.  The  Greenland  gla- 
cier covers  an  area  of  over  500,000  square  miles ;  and  the 
Antarctic  ice  sheet  is  several  times  greater  than  this. 

From  the  immense  size  of  the  icebergs  that  float  away 
from  the  margin  of  the  Antarctic  ice  sheet,  we  are  certain 
that  the  depth  of  this  glacier  is  greater  than  a  mile;  and 


314 


PHYSICAL   GEOGBAPHY, 


there  is  some  reason  for  thinking  that  it  is  nearly  two  miles 
in  deptli,  even  at  the  margin,  while  in  the  interior  the 
depth  may  be  over  five  miles.  But  about  the  actual 
conditions  existing  on  this  sheet  of  ice  we  have  very  little 
knowledge,  for  this  part  of  the  world  is  almost  entirely 
unexplored. 

Within  a  few  years,  our  information  concerning  the  Green- 
land ice  sheet  has  become  very  much  increased.  Several 
parties  have  examined  it  along  the  coast,  and  others  have 
passed  into  the  interior  of  the  Greenland  continent.     Near 

the  margin,  the 
ice  extends  down 
to  the  sea,  some- 
times as  a  solid 
wall,  but  usually 
in  the  form  of 
tongues  extend- 
ing down  the 
valleys.  The  ice 
front  is  often 
hundreds  of  feet 
in  height,  and 
when  it  extends 
into  the  ocean, 
bergs  are  fre- 
quently detached  and  floated  away.  Passing  from  this 
rather  irregular  margin  toward  the  interior,  there  is  an 
area  of  rough  ice  which  is  difficult  to  traverse,  and  through 
which  there  are  some  projecting  mountain  peaks,  known  to 
the  Greenlanders  as  nunataks  (Fig.  181).  These  rise  above 
the  great  ice  field  as  the  only  parts  of  the  land  exposed  to  the 
air.  Beyond  a  few  miles  from  the  coast,  even  these  high 
mountain  peaks  disappear,  and  there  is  a  great  ice  plateau, 


Fig.  181. 
A  nunatak  rising  above  the  Greenland  ice  sheet. 


GLACIERS. 


315 


generally  over  a  mile  above  the  sea,  and  in  some  cases  hav- 
ing an  elevation  of  about  10,000  feet. 

Whatever  the  topography  of  Greenland  may  be,  this 
immense  sheet  of  ice  entirely  obscures  it,  and  it  probably 
covers  a  land  which  is  mountainous  in  character.  The  sur- 
face of  the  ice  in  the  interior  is  very  smooth,  and  one  may 
travel  over  it  with  considerable  ease.  The  movement 
appears  to  be  in  all 
directions,  from  the 
central  part  toward 
the  sea,  as  if  the 
accumulation  were 
greater  in  the  in- 
terior than  else- 
where. We  can 
form  no  idea  con- 
cerning the  depth 
of  this  sheet  of 
ice ;  but  it  is  a 
moderate  estimate 
to  say  that  it  is  cer- 
tainly several  thou- 
sand feet  in  depth. 

Icebergs.  —  The 
cold  Arctic  winter 
causes    the     ocean 

surface  to  become  frozen ;  and  the  movement  of  the  waters, 
resulting  from  the  winds,  currents,  and  tides,  often  breaks 
this  ice  and  throws  it  into  hummocks,  so  that  during  this 
season  the  Arctic  water  presents  a  rough  ice  surface.  Dur- 
ing the  summer  this  partly  or  entirely  breaks  up,  and  the  ice 
either  melts  or  floats  away.  Added  to  this  floe  ice^  are  the  ice- 
bergs which  are  derived  from  the  margins  of  glaciers  extend- 


FiG.  182. 
Icebergs  in  the  Antarctic. 


316 


PHYSICAL   GEOGRAPHY. 


=^^^^^^^^^£^ Sea  Level 


ing  into  the  ocean  (Fig.  182).  As  the  ice  moves  into  the  sea, 
the  buoyancy  of  the  water  causes  it  to  break  into  fragments, 
which  then  drop  into  the  ocean  and  drift  away.  Carried  by 
the  currents,  these  bergs  may  pass  hundreds  of  miles  from 
their  source ;  and  the  Atlantic  steamers  not  uncommonly 
encounter  large  icebergs  that  have  been  derived  from  the 
Greenland  glaciers,  while  upon  the  shores  of  Newfoundland 
these  are  often  stranded.  An  iceberg  is  mostly  beneath  the 
water;  for,  in  a   regularly  formed  ice  block,  there  are  8.7 

parts  below  the  surface  of  the 
water  for  every  one  part  that 
is  above.  Therefore  if  an  ice- 
berg of  regular  form  projects 
100  fee  5  into  the  air,  there  are 
870  feet  below  the  surface  of 
the  water  (Fig.  183).  In  the 
case  of  irregular  icebergs,  this 
may  not  be  true.  The  icebergs 
from  the  Greenland  glacier 
often  extend  to  a  height  of  100 
or  200  feet ;  but  those  from 
the  Antarctic  ice  sheet  are 
sometimes  several  hundred  feet 
above  the  surface.  Some  bergs  have  been  reported  in  the 
Antarctic,  which  had  a  height  of  over  500  or  600  feet  above 
the  water.  One  such  berg  extended  to  the  height  of  580 
feet  above  the  sea,  and  had  a  length  of  nearly  three  miles, 
so  that  the  captain  who  saw  it  believed  it  to  be  an  island. 
Other  cases  of  icebergs  with  a  length  of  over  a  mile,  and  a 
height  of  more  than  500  feet,  have  been  reported  from  this 
region.  Such  bergs  measure  about  a  mile  from  the  top  to  the 
bottom  which  is  beneath  the  sea. 

Glacial  Period :  Area  covered  hy  Ice.  —  As  Avas  stated  in  the 


Fig.  183. 

Diagram  to  show  relative  proportion 

of  submerged  ice  in  an  iceberg. 


GLACIERS. 


317 


last  part  of  Chapter  VII.,  the  climatic  conditions  which  we 
noAv  find  upon  the  earth,  have  not  always  been  the  same. 
The  most  recent  and  pronounced  climatic  changes,  were 
those  which  caused  the  extension  of  arctic  conditions  into 
parts  of  the  north  temperate  zone,  and  then,  later,  a  change 
from  this  condition  to  the  present  temperate  climate.     As  a 


Fig.  184. 
Glacial  lakes  and  moraine,  in  a  mountain  valley  not  now  occupied  by  a  glacier. 

result  of  these  changes,  the  so-called  glacial  period  was 
caused.  This  expressed  itself  in  an  increase  in  snow,  both 
among  the  high  mountains  of  the  temperate  zone,  and  in  the 
higher  latitudes.  The  valley  glaciers  of  Switzerland,  the 
Sierra  Nevadas,  and  other  mountains,  were  more  extensive 
than  at  present,  and  mountain  chains  in  which  there  are  now 
no  glaciers,  then  had  their  valleys  filled  with  ice  streams 


318  PHYSICAL   GEOGRAPHY, 

(Fig.  184).  But  the  most  remarkable  effect,  was  the  pro- 
duction of  ice  sheets  of  thoroughly  continental  character, 
both  in  northwestern  Europe  and  in  northeastern  America. 

The  entire  north  temperate  zone  does  not  seem  to  have 
been  occupied  by  a  glacier,  but  there  appear  to  have  been 
several  large  sheets,  one  set  in  Europe  and  another  in 
America.  It  is  not  certain  whether  these  were  connected 
with  the  Greenland  glaciers,  but  there  seems  reason  to  doubt 
whether  there  was  such  a  connection.     The  extension  of  the 


Fig.  185. 
Approximate  extension  of  the  continental  ice  sheet. 

glacier  in  the  United  States  is  shown  on  the  map  (Fig.  185). 
The  entire  region  north  of  the  line  indicating  the  terminus 
of  the  ice,  was  covered  with  a  glacier  which  appears  to 
have  resembled  in  most  respects  that  which  we  now  find 
on  Greenland.  Off  the  New  England  coast  the  ice  entered 
the  ocean,  and  from  it  icebergs  were  discharged ;  but  in  the 
interior,  the  ice  front  appears  to  have  changed  in  position 
from  time  to  time,  now  advancing,  now  retreating.  Near 
the  margin,  where  the  country  was  mountainous,  the  higher 
hills  projected  above  the  ice  in  a  manner   similar  to   that 


GLACIERS.  319 

noticed  along  the  margin  of  the  Greenland  glacier  ;  but  in 
the  interior  of  the  ice  sheet,  the  highest  mountains  appear 
to  have  been  entirely  buried.  There  is  evidence  that  the 
White  Mountains  of  New  Hampshire,  the  Green  Mountains 
of  Vermont,  and  the  Adirondacks  of  New  York  were  all 
enveloped  in  this  sheet  of  ice. 

In  Europe  the  conditions  appear  to  have  been  similar,  and 
the  greater  part  of  the  British  Isles,  Scandinavia,  Russia, 
and  Germany,  were  covered  with  an  ice  sheet,  or  perhaps 
with  several  great  glaciers  moving  from  different  centers. 
Recent  studies  seem  to  show  that  the  Greenland  ice  did  not 
have  a  much  greater  extension  then  at  present,  and  that  the 
region  between  America  and  Europe  Avas  not  filled  with  ice. 
So  far  as  we  have  evidence,  there  are  no  signs  of  extensive 
glaciation  in  northern  Asia ;  nor  was  there  on  the  west  coast 
of  America,  an  ice  sheet  which  in  point  of  size  would  com- 
pare with  that  of  eastern  United  States  and  Canada. 

Why  the  climate  changed,  cannot  be  said ;  and  all  that  we 
can  state  definitely  is,  that  we  know  that  there  was  this 
change.  We  are  not  certain  how  long  the  ice  remained,  nor 
when  it  came,  nor  what  its  detailed  history  was.  We  do 
know,  that  before  the  glacial  period,  the  climate  was  not 
frigid ;  that  the  ice  occupied  the  regions  for  a  considerable 
length  of  time  ;  and  that  since  then,  the  conditions  have  again 
become  temperate.  Studies  of  the  rate  of  formation  of  such 
gorges  as  those  of  Niagara,  and  the  Mississippi  below  the 
falls  of  St.  Anthony,  which  began  when  the  ice  retreated, 
lead  to  the  conclusion  that  the  close  of  the  glacial  period 
was  probably  between  7000  and  10,000  years  ago.  From  the 
geological  standpoint,  it  was  one  of  the  most  recent  chapters 
in  the  history  of  the  world. 

Terminal  Moraine. — The  continental  ice  cap  of  the  glacial 
period  behaved  very  much  as  the  Greenland  ice  sheet  does 


320  PHYSICAL   GEOGRAPHY. 

at  present.  Since  no  land  projects  above  it,  the  Greenland 
glacier  is  not  able  to  carry  morainal  material  upon  its  sur- 
face ;  and  the  same  appears  to  have  been  true  of  the  conti- 
nental glaciers  of  the  United  States  and  Europe.  Like  the 
Greenland  glacier,  each  of  these  ice  sheets  moved  from  some 
central  region,  in  case  of  eastern  America  apparently  from 
the  region  of  Hudson  Bay  or  Labrador  ;  and  as  they  moved, 
they  dragged  rock  material  from  northern  towards  southern 
regions.  When  the  ice  disappeared,  much  of  this  material 
was  left,  just  as  would  be  the  case  if  the  Greenland  glacier 
should  melt  away.    As  in  the  Greenland  and  valley  glaciers, 

the  front  margin  of  the  ice 
W  was  a  place  of  wastage,  at 

which  much  material  was 
accumulated  in  the  form 
of  a  terminal  moraine. 
One  of  the  most  distinct 
terminal  moraines  formed 
by  the  glacier  of  the 
^^''-  ^^^^-  United  States,  follows  the 

Boulder  in  the  moraine  at  Cape  Ann,  Mass.     ,  .,        i      i     i    t  ,^ 

heavily  shaded  line  on  the 
map  (Fig.  185).  Other  moraines  are  found  north  of  this, 
marking  stages  of  halting  during  the  retreat  of  the  ice. 

Both  in  Europe  and  America,  the  glacier  has  produced  a 
very  pronounced  effect  upon  the  topography  and  the  condi- 
tions of  the  land  surface.  There  are  many  details  which  it 
would  be  impossible  to  consider  in  a  work  of  this  kind ;  but 
some  of  the  more  pronounced  features  may  be  mentioned. 
The  terminal  moraine  is  one  of  the  most  striking  topo- 
graphic forms  resulting  from  glacial  action.  The  topography 
is  extraordinarily  rough  and  irregular.  There  are  hills  and 
hummocks,  enclosing  valleys  and  pits;  and  all  are  thrown 
together  in  the  most  confused  manner.     The  material  com- 


GLACIERS. 


321 


posing  them  is  partly  clay,  partly  gravel ;  and  fragments  of 
all  sizes,  from  tiny  bits  of  clay  to  large  boulders  (Fig.  186), 
are  confusedly  thrown  together.     Sometimes  the  surface  of 


Fig.  187. 

The  bear  den  moraine  at  Cape  Ann,  Mass.,  — a  moraine  whose  surface  is  covered 

with  boulders. 

the  moraine  is  strewn  with  large  boulders  (Fig.  187),  and 
the  morainal  material  is  often  100  or  200  feet  in  depth,  and 
sometimes  even  more. 

Formation  of  Soil.  — The 
ice  contained  much  rock 
material  derived  from  more 
northern  regions;  and  when 
it  ceased  to  move,  and 
melted  away,  this  was 
dropped  at  the  place  which 
it  had  reached.  This 
ground  moraine,  which  is 
commonly  known  as  till 
or  boulder  elay^  forms  the  soil  of  the  greater  part  of  the 


Fig.  188. 

Boulder-strewn  till  soil  in  Maine.  Many 
boulders  taken  from  the  surface  and 
built  into  walls. 


322 


PHYSICAL   GEOGBAPHY. 


country  included  within  the  glacial  limits.  It  is  a  clay 
through  which  boulders  of  various  sizes  are  scattered 
(Fig.  188) ;  and  these  boulders  may  often  be  recognized  as 
fragments  derived  from  hills  to  the  north,  while  the  finer 
particles  are  the  result  of  the  grinding  action  of  the  moving 
ice.     For  instance,  in  central  New  York  many  of  the  bould- 


FiG.  189. 
A  limestone  pebble  covered  with  glacial  scratches. 


ers  have  come  from  the  Canadian  highlands.  The  scouring 
action  that  was  in  progress  beneath  the  ice,  is  shown  by  the 
fact  that  these  boulders  and  pebbles  are  finely  scratched  and 
grooved  (Fig.  189);  and  the  same  is  true  of  the  bed  rock 
beneath  the  soil.  At  times  this  till  soil  is  several  hundred 
feet  in  thickness,  but  usually  its  depth  is  only  a  few  feet. 
With  the  melting  of  the  ice,  streams  were  furnished  both 


GLACIERS.  323 

with  increased  quantities  of  water,  and  with  increased  sup- 
plies of  sediment  ;  and  these  swollen  rivers  carried  away 
from  the  ice  a  large  part  of  the  rock  material  which  it  bore, 
depositing  some  in  their  valleys,  and  spreading  some  of  it 
over  the  lowlands.  In  part,  at  least,  the  prairie  soil  of  some 
of  the  Central  States  appears  to  be  due  to  this  action  of  ice 
melting ;  and  the  teri^aces  of  many  of  the  streams  that 
extended  from  the  ice  front,  have  been  derived  in  the  same 
manner.  Even  a  part  of  the  delta  of  the  Mississippi  is 
probably  built  of  sediment  furnished  by  the  melting  ice, 
when  the  front  of  the  glacier  stretched  across  the  head- 
water tributaries  of  this  river. 

Formation  of  Lakes. — Temporary  lakes  were  formed  by 
the  ice,  and  in  one  or  two  cases  these  were  of  great  size. 
They  were  commonly  formed  where  the  ice  extended  across 
streams  that  flowed  toward  the  north,  thus  acting  as  a  dam, 
and  preventing  them  from  taking  their  normal  courses. 
While  hundreds  of  such  lakes  were  caused,  one  that 
formed  in  the  valley  of  the  Red  River  of  the  North  was 
by  far  the  largest  and  most  remarkable  of  all.  This  lake, 
which  has  now  disappeared,  at  one  time  covered  an  area  of 
110,000  square  miles,  being  15,000  square  miles  greater  than 
the  five  Great  Lakes  combined.  It  covered  the  area  included 
within  the  great  wheat  belt  of  the  Red  River  valley,  in 
Minnesota,  North  Dakota,  and  Manitoba  ;  and  Lakes  Mani- 
toba, Winnepeg,  and  Winnipegosis  are  descendants  of  this 
great  lake,  their  combined  area  at  present  being  but  12,500 
square  miles. 

Lake  Agassiz,  as  this  great  temporary  water  body  is  called, 
at  places  had  a  depth  of  500  or  600  feet,  and  it  outflowed 
southward,  over  the  divide  in  Minnesota,  entering  the  Minne- 
sota River,  and  passing  thence  into  the  Mississippi.  Thus 
by  this  great  ice  dam,  drainage  which  now  finds  its  escape 


324 


PHYSICAL   GEOGBAPHY. 


into  the  Arctic,  was  forced  to  flow  in  the  opposite  direction 
and  enter  the  Gulf  of  Mexico.  The  proof  of  the  existence 
of  this  great  lake,  is  found  partly  in  the  presence  of  beaches 
and  wave-cut  cliifs,  now  standing  high  above  the  bottom  of 
the  valley,  and  partly  in  the  great  level  plain  of  the  Red 
River  valley  (Fig.  215).     The  levelness  of  this  plain  is  due 

to  the  deposit  of  sedi- 
ment in  the  lake,  the 
bottom  being  some- 
what like  that  of  Lake 
Erie. 

Among  the  other 
striking  effects  of  the 
glacial  period,  was  the 
formation  of  many  of 
the  existing  lakes.  In 
Minnesota  there  are 
fully  10,000  lakes  and 
ponds  which  were 
caused  by  the  glacier; 
and  throughout  the 
Northern  States,  there 
are  scores  of  thousands 
of  glacial  lakes  (Fig. 
190).  Before  the  ice 
occupied  the  country, 
the  rivers  had  well- 
established  drainage 
lines,  and  pronounced 
valleys  existed.  For  a  time  the  ice  occupied  these  and 
prevented  them  from  being  used  as  drainage  ways.  When 
the  glacier  melted,  it  deposited  the  rock  materials  which 
it   was   carrying,    and    deposited   these    regardless    of    the 


SCALE  OF  .MILES 

r  I I , I  t 

0         12        3        4         5 

Fig.  190. 
Map  of  a  part  of  Massachusetts,  showing  abun- 
dance of  lakes  caused  by  glacial  conditions. 
Shaded  areas  represent  lakes. 


GLACIERS.  325 

pre-glacial  drainage  lines.  Sometimes  great  masses  were 
dumped  across  a  stream  channel,  while  in  other  cases,  as  for 
instance  upon  plains,  the  glacial  materials  were  deposited 
irregularly,  so  that  basins  were  formed  on  the  drift-covered 
surface.  Also,  during  its  movement,  the  ice  appears  to 
have  deepened  some  valleys  more  than  others,  and  some  parts 
of  valleys  more  than  other  portions,  thus  forming  rock  basins. 
All  of  these  basins,  Avhatever  their  cause,  were  filled  with 
standing  water  when  the  ice  melted,  and  were  thus  trans- 
formed to  lakes.  When  the  glacier  disappeared,  the  surface 
of  the  land  was  dotted  with  lakes  of  various  sizes  and  depths, 
and  many  of  them  still  remain  (Figs.  168  and  190),  although 
some  of  the  smaller  have  been  destroyed,  or  transformed  to 
swamps  (Fig.  172),  either  by  filling  or  by  cutting  down  the 
gravel  barrier.  Even  the  Great  Lakes  appear  to  owe  their 
origin,  in  large  part,  if  not  entirely,  to  the  action  of  the  ice ; 
and  the  same  is  true  of  the  Finger  Lakes  of  central  New 
York,  of  Lake  Champlain,  and  indeed  of  practically  all  the 
lakes  north  of  the  terminal  moraine. 

Formation  of  Waterfalls.  — As  a  result  of  the  same  cause,  the 
streams  which  began  to  flow  after  the  ice  disappeared,  were 
often  on  one  side  of  their  pre-glacial  channels.  Some  were 
entirely  turned  out  of  their  valleys  and  forced  to  form  new 
ones.  Others  were  only  turned  aside  for  short  distances  ; 
and  in  some  extreme  cases,  they  were  actually  caused  to  flow 
over  old  divides,  in  an  opposite  direction  from  that  which 
they  had  pursued  before  the  beginning  of  the  glacial  period. 
The  time  that  has  elapsed  since  the  close  of  the  glacial  period 
is  very  brief  considered  from  the  geological  standpoint ;  and 
for  this  reason,  the  streams  that  have  been  obliged  to  cut  new 
valleys  have  succeeded  in  producing  only  very  narrow  gorges 
(Frontispiece,  and  Figs.  133  and  191).  The  action  of  cut- 
ting  in   the   channel   has    exceeded    that    of     weathering, 


326 


PHYSICAL   GEOGBAPHT. 


and  these  young  valleys  are  narrow,  steep-sided,  canon- 
like gorges,  in  which  waterfalls  are  common.  We  find 
illustrations  of  these  post-glacial  valleys  in  almost  every 
part  of  the  region  occupied  by  the  ice.  Side  by  side  we  may 
often  see  the  pre-glacial  valley,  with  its  broad,  gently  sloping 
sides,  and   the   narrow,  gorge-like   channel  of    post-glacial 

origin.  These  may  often  be  found 
in  the  same  valley,  the  stream  for 
part  of  its  distance  occupying  its 
pre-glacial  course,  and  in  places  be- 
ing in  these  post-glacial  trenches. 

So  pronounced  has  been  the  effect 
of  the  ice  in  the  production  of  lakes 
and  waterfalls,  that  with  a  fair  de- 
gree of  accuracy  one  could  map  the 
southward  extension  of  the  ice  sheet 
by  merely  drawing  a  line  across  the 
country,  separating  the  region  of 
abundant  lakes,  waterfalls  and 
gorges,  from  the  regions  to  the 
south,  in  which  these  features  are 
rare,  if  not  entirely  absent.  This 
is  particularly  well  illustrated  in 
New  Jersey  where  the  line  runs  in 
a  westerly  direction ;  and  one  can 
see  the  point  well  brought  out  by 
examining  a  map  of  a  part  of  Massachusetts,  New  York, 
Wisconsin,  Minnesota,  etc.,  and  comparing  it  with  a  similar 
map  of  Kentucky,  Virginia,  etc.  The  entire  drainage  sys- 
tem of  the  land  that  was  covered  by  the  ice  has  been  rejuve- 
nated, and  the  details  of  topography  have  often  been  en- 
tirely altered.  The  great  features  of  hills  and  valleys  are 
practically  the  same   as  those  which  existed  before  the  ice 


Fig.  191. 

A  view  in  Watkins  Glen,  New 

York,  —  a  post-glacial  gorge. 


GLACIEBS,  327 

came;    but  many  of  the  minor  details  of  sculpturing  and  of 
tilling  are  the  result  of  glacial  or  post-glacial  changes. 


REFERENCE   BOOKS. 

Wright.  — The  Ice  Age  in  North  America.  Appleton  &  Co.,  New  York. 
Third  edition,  1891.  8vo.  $5.00.  (From  the  standpoint  of  the  American 
student,  the  best  book  on  the  subject.) 

Wright.  —  Man  and  the  Glacial  Period.  Appleton  &  Co.,  New  York. 
(International  Scientific  Series.)  1892.  12mo.  $1.75.  (Partly  based 
on  "The  Ice  Age,"  being  a  smaller  but  very  similar  book.) 

The  subject  of  British  Glacial  Geology  is  treated  by  Geikie,  "  The 
Great  Ice  Age."  Stanford,  London.  (Appleton  &  Co.,  New  York  agents.) 
Third  edition.     Kevised,  1894.     8vo.     $7.50. 

See  also  Lewis,  "The  Glacial  Geology  of  Great  Britain  and  Ireland." 
Longmans,  Green,  &  Co.,  New  York,  1894.     8vo.     $7.00. 

For  Canadian  Glaciation,  see  Dawson,  "The  Canadian  Ice  Age." 
Scientific  Publishing  Co.,  New  York,  1894.     12mo.     $2.00. 

Much  valuable  information  and  many  illustrations  are  contained  in 
Shaler  and  Davis,  "  Illustrations  of  the  Earth's  Surface,  Glaciers."  For 
sale  by  Houghton,  Mifflin  &  Co.,  Boston,  1881.     4to.     $10.00. 

For  Moraines,  see  Chamberlin,  Third  Annual  Report,  U.  S.  Geological 
Survey,  1883. 

For  Glacial  Striations,  see  Chamberlin,  Seventh  Annual  Report  of  the 
same,  1888. 

For  Alaskan  Glaciers,  see  Russell,  Thirteenth  Annual  Report  of  the 
same,  1893. 

For  Existing  Glaciers  of  the  United  States,  see  Russell,  Fifth 
Annual  Report  of  the  same,  1885. 

For  a  statement  of  Croll's  Hypothesis  for  the  cause  of  the  glacial 
period,  see  his  "  Climate  and  Time,"  referred  to  at  the  end  of  Chapter  VII. 


CHAPTER   XVIII. 


THE   COAST  LINE. 


General  Statement.  —  The  seacoast  is  a  place  of  very 
active  change,  for  here  a  very  slight  movement  in  the  land 
registers  itself  distinctly  in  the  outline  of  the  shore.  Mate- 
rials are  being  brought  by  various  agents  and  deposited  in 
the  sea ;  and  along  the  shore  line,  there  are  ever-acting  forces 
which  tend  to  wear  back  the  coast  and  change  the  outline.  The 
agents  of  destruction  are 
mainly  those  of  waves  and 
associated  currents ;  and 
the  materials  removed 
from  the  coast  by  wind 
waves,  are  taken  away  and 
distributed  over  the  sea 
bottom  by  wind,  tidal  and 
ocean  currents.  There  is 
very  little  difference  be- 
tween the  coast  line  fea- 
tures of  the  sea  and  those 

of  lakes.  Waves  repeat  on  the  lake  shores  nearly  all  the 
features  of  the  ocean  shore  line  (compare  Fig.  192  with 
212,  and  200  with  213),  though  usually  with  less  intense 
development.  Cliffs  and  headlands  are  less  pronounced, 
beaches  are  less  extensive,  the  action  of  tides  is  absent,  and 
in  many  minor  ways  the  lake  shore  lines  differ  from  those 
of  the  sea;  but  in  general  features  there  is  a  close  resem- 
blance. 

328 


Fig.  192. 

A  cliff  at  Cape  Cod,  Mass.,  showing  de- 
structive action  of  waves. 


THE  COAST  LINE.  329 

Effect  of  Elevation.  —  Since  the  sea  bottom  is  mostly  level, 
and  since  deposits  of  unconsolidated  sediment  are  spread 
over  it  (see  pages  157  and  164),  an  elevation  of  the  bottom 
above  sea  level,  usually  produces  a  regular  coast  line,  and 
the  materials  composing  the  coast  are  soft  clays  or  sands. 
There  is  a  general  absence  of  projecting  capes,  promontories, 
islands,  and  the  smaller  irregularities  of  the  coast.  The  kind 
of  shore  line  that  is  produced  by  this  cause,  is  well  illustrated 
on  the  coast  of  Texas  (Fig.  194),  although  here  there  have 
been  some  irregularities  introduced  by  other  causes.  Great 
sandy  beaches,  extending  for  many  miles,  separate  the  dry 
land  from  the  sea ;  there  are  no  rocks  and  no  high  cliffs,  but 
sand  everywhere. 

Effect  of  Depression.  —  The  effect  of  depression  of  the 
land,  or,  what  would  amount  to  the  same  thing,  the  elevation 
of  the  sea  level,  produces  just  the  opposite  result.  Instead 
of  causing  a  regular  coast  line,  it  produces  marked  irregulari- 
ties. If  the  student  could  imagine  the  sea  rising  to  the  level 
of  the  place  upon  which  he  lives,  he  would  have  some  idea 
of  the  coast  irregularities  that  would  result  from  a  depres- 
sion of  the  land.  The  sea  would  rise  to  the  perfectly  hori- 
zontal line,  and  would  extend  up  every  valley  to  the  supposed 
level.  Some  low  hills  would  be  entirely  submerged,  while 
others  that  rose  to  heights  slightly  above  the  new  sea  level, 
would  form  islands.  Projecting  hills  would  be  transformed 
to  promontories  or  capes,  and  the  stream  valleys  would  either 
become  estuaries  or  bays  (Fig.  193). 

In  many  parts  of  the  world,  the  last  change  at  present  dis- 
tinctly registered  along  the  coast,  has  been  that  of  submer- 
gence of  the  land ;  and  in  such  places  we  find  an  exact  repro- 
duction of  the  conditions  imagined.  If  one  examines  the 
coast  of  Maine,  as  represented  upon  a  good  map,  it  is  readily 
seen  that  the  numerous  bays  and  islands  are  nothing  but  land- 


330 


PHYSICAL   GEOGBAPHT. 


Fig.  193. 

Coast  of  Mt.  Desert,  Maine,  showing  effect 
of  submergence. 


made  forms  which  have  been  partly  submerged  beneath 
the  sea.  Figure  211,  representing  a  part  of  this  coast, 
is   a   particularly  good  illustration  of  these    irregularities. 

The  coast  of  northern  Eu- 
rope illustrates  the  same 
type;  and  on  the  American 
coast,  not  merely  does 
Maine  furnish  an  illustra- 
tion, but  from  the  Arctic 
to  Florida,  there  are  abun- 
dant instances  of  this 
same  effect  of  land  move- 
ment. Chesapeake  Bay 
(Plate  24)  is  a  land-made 
valley  into  which  the  sea 
has  entered  by  submer- 
gence ;  and  the  tributaries  to  this  bay  are  river  valleys  also 
partly  drowned  by  the  sea.  Those  who  dwell  upon  these 
coasts  find  it  impossible  to  say  where  the  river  ends  and 
the  sea  begins. 

Thus  elevation  tends  to  produce  smooth  coasts,  while  de- 
pression introduces  irregularities;  and  since  the  crust  of  the 
earth  is  in  almost  constant  movement,  either  in  one  or 
the  other  of  these  ways,  we  find  that  the  general  outline  of 
the  seacoast  is  usually  either  very  irregular  or  very  smooth. 
In  this  connection  one  may  well  compare  the  northeastern 
coast  of  the  United  States,  where  the  land  has  recently  been 
lowered,  with  the  western  coast  of  South  America,  where 
the  land  is  rising. 

Effect  of  Sediment.  —  The  waves  and  currents  in  the  sea, 
tend  to  distribute  over  the  sea  bottom  all  mechanical  frag- 
ments brought  to  them  from  the  land,  and  to  form  sedimentary 
deposits  with  them.  AGenerally  the  sea  is  able  to  remove 


THE  COAST  LINE. 


331 


these  materials  and  to 
deposit  them  away 
from  the  coast ;  but 
in  some  cases,  the 
amount  of  sediment 
brought  exceeds  the 
ability  of  the  oceanic 
agents  to  remove  it. 
This  is  particularly  the 
case  at  the  mouths  of 
large  rivers  where 
deltas  are  being 
formed.  Thus  oppo- 
site the  mouth  of  the 
Mississippi,  the  coast 
is  rapidly  growing  out- 
ward in  the  form  of  a 
delta  (Fig.  153),  and 
the  same  is  true  of 
the  Nile,  and  manv 
other  large  rivers  of 
the  world.  Even 
where  this  is  not  hap- 
pening, the  amount  of 
sediment  brought  to 
the  sea  may  so  far  ex- 
ceed the  power  of  the 
waves  to  remove  it, 
that  the  coast  grows 
outward.  Very  nearly 
the  entire  coast,  from 
Sandy  Hook  to  the 
northern  boundary  of 


'Brazos  Santiago 


my  ■ 


y 


J 1  ■ 


^v; 


/^V"^. 


Ba^dii 


(.'' 


S"^ 


Rio 
'Grande 


SCALE  OF  Miles 

'      '      '      '      '      ' 
0     12     3     4     5 

B.D.Stnv,  N.T. 


Part 


Fig.  194. 
of  an  extensive  sand  bar  on  the  Texas  coast. 


332 


PHYSICAL   GEOGEAPHT. 


Florida,  is  being  built  outward  by  the  accumulation  of 
sediment  that  the  waves  have  not  been  able  to  distribute 
over  the  sea  bottom.  This  sediment  is  brought  to  the  sea 
by  the  rivers,  and  is  piled  by  the  waves  into  sand  banks  and 
bars ;  and  these  bars  extend  as  long  islands  parallel  to  the 
coast  (Fig.  194),  being  separated  from  the  mainland  by 
shallow  bodies  of  water  in  which  salt  marshes  are  often 
present. 

Effect  of  Waves  and  Currents.  —  On  exposed  coasts,  the 


"ivmmjvfM  '■*"y 


Fig.  195. 

View  of  the  island  of  Heligoland,  and  map  showing  how  rapidly  it  is  being 
destroyed.  Outside  line  shows  boundary  in  the  year  800,  when  the  circumfer- 
ence was  120  miles ;  shaded  area,  boundary  in  1300  (circumference,  45  miles) . 
Innermost  area,  8  miles  in  circumference. 


ocean  waves  are  constantly  beating  with  such  force  that 
even  the  very  hardest  of  rocks  are  worn  away.  On  the 
European  shore,  within  historic  times,  this  destruction  by 
the  waves,  combined  with  the  action  of  the  tides  in  remov- 
ing the  fragments,  has  caused  the  coast  to  retreat,  often  for 
distances  of  several  miles.  Places  that  a  few  hundred  years 
ago  were  at  a  considerable  distance  from  the  coast,  are  now 
either  entirely  destroyed,  or  else  are  nearer  the  sea  than  for- 
merly. On  parts  of  the  coast  of  England,  the  sea  cliffs  are 
being  worn  back  at  the  rate  of  five  or  six  feet  a  year ;  and 


THE  COAST  LINE. 


333 


i'lG.  ilio. 
Lake  spit. 


it  has  been  estimated  that,  on  a  part  of  the  coast  of  York- 
shire, the  shore  line  has  been  worn  back  a  distance  of  two 
miles  since  the  time  of  the  Norman  Conquest.     Many  simi- 
lar  cases   might   be  intro-     ^^53^^.^^^^^^^^^^^^^^^^^^^^.^^.^^.^ 
duced  in  illustration  of  this 
wearing  back  of  the  coast 
line    (Fig.    195).     On  the 
American    coast,    we   have 
no  remarkable  instances  of 
rapid     change ;     but     still 
there   is    every  reason  for 
believing    that  the  shores, 
in   certain  exposed  places, 
are    actually    being    worn 
back  at  a  perceptible  rate. 
At  the  southern  end  of  Martha's  Vineyard,  the  cliffs  of  Gay 
Head  are  thus  retreating. 

While  in  some  places  the   action  of  waves  and  currents 

is  destroying  the 
coasts,  in  others 
it  is  engaged  in 
building  them  up. 
This  was  stated  in 
the  preceding  sec- 
tion; and  not  only 
is  it  true  in  that 
large  way,  but  also 
in  a  small  way. 
The  tidal  currents 
in  the  vicinity  of 
Nantucket  and  Martha's  Vineyard,  on  the  south  side  of 
Massachusetts,  are  moving  the  sands  in  such  a  way  that  bars 
are  being   formed,  and  are  almost  constantly  changing  in 


Fig.  197. 
Hook,  Lake  Michigan. 


834 


PHYSICAL   GEOGRAPHY. 


size  and  position.  In  some  places,  where  the  direction  of 
the  currents  is  favorable,  permanent  bars,  or  spits,  are  built 
out  from  the  land  (Fig.  196).  Sometimes  they  are  curved; 
and  such  sand  bars  are  known  as  hooks  (Fig.  197). 

According  to  the  conditions  under  which  they  are  work- 
ing, there  is  a  very  marked  difference  in  the  action  of  these 
oceanic  agents.  On  exposed  headlands  which  jut  into  the 
sea,  the  action  of  waves  is  violent,  and  the  coast  line  in  such 


Fig.  198. 
Sea  cave  in  a  well-jointed  granite  rock,  Cape  Ann,  Mass. 

places  is  liable  to  be  very  precipitous.  In  enclosed  or  par- 
tially enclosed  estuaries  and  bays,  the.  wind  waves  are  of 
very  little  importance,  and  the  changes  of  the  coast  line  are 
relatively  moderate.  In  harbors,  for  instance,  the  wind 
waves  are  producing  almost  no  change  in  the  coast.  Such 
enclosed  areas  are  usually  the  seat  of  deposition,  instead  of 
places  in  which  destructive  action  is  in  progress. 

Great  difference  also  results  with  variations  in  the  kind  of 
rock  which  makes  the  coast.     The  waves  find  it  very  easy 


THE  COAST  LINE. 


835 


to  cut  their  way  into  soft  sand  and  clay,  while  hard  granite 
rocks  resist  their  action.  In  the  hard  massive  rocks,  par- 
ticularly if  these  are  exposed  to  the  action  of  the  oceanic 
waves,  there  are  produced 
cliffs  of  great  size  and 
ruggedness.  Against  the 
base  of  these,  the  waves 
dash  with  violence ;  and 
along  the  line  at  which 
they  are  wearing,  sea  caves 
are  often  cut  in  the  rock 
(Fig.  198).  The  cliff  is 
undermined  along  this  line 
of  wave  action;  and,  by  the 
dropping  down  of  frag- 
ments, it  tends  to  remain  in  the  form  of  a  cliff.  Where  the 
rocks  of    the   coast  are  soft,  these  very  precipitous  slopes 


Fig.  199. 

Indentation  on  the  coast  of  Cape  Ann, 
Mass.,  where  the  waves  are  removing  a 
soft  dike  rock  which  crosses  the  hard 
granite. 


Fig.  200. 
Pond  enclosed  behind  a  beach  which  is  built  across  a  small  bay.  Cape  Ann,  Mass. 


336 


PHYSICAL   GEOGBAPHY, 


Fig.  201. 
A  crescent-shaped  beach,  Cape  Ann,  Mass, 


cannot  be  maintained ;  and  where  a  hard  rock  is  crossed  by 

a  less  durable  one,  the  coast  is  rendered  irregular  (Fig.  199). 

While   these   peculiarities   of    coast   line   may  be    found 

developed  in  many  parts  of  the  earth,  the  tendency  of  the 

waves  and  currents  is  to 
render  the  coast  line  al- 
ways more  regular.  Ma- 
terials are  worn  by  the 
waves  from  the  headlands, 
and  drifted  into  the  bays, 
which  they  tend  to  fill. 
In  the  course  of  time,  if 
nothing  interferes,  this 
material  is  formed  as  a  bar 
across  the  mouth  of  the 
bays,  and  later  is  built  into  a  beach,  which  rises  above  the 
surface  of  the  water,  enclosing  the  bay  as  a  pond  behind 
the  beach  barrier  (Figs.  200  and  213).  The  material  built 
into  the  beach  is  usually  de- 
posited in  the  form  of  seg- 
ments of  a  circle,  concave 
toward  the  sea,  giving  the 
well-known  crescent-shaped 
beach  of  the  seashore  (Fig. 
201).  The  headlands  form 
the  two  ends  of  the  seg- 
ments, and  the  material  on 
the  beach  grades  from  coarse 
pebbles  (Fig.  202)  near 
the  headlands,  to  fine  sand  in  the  central  part  of  the  beach. 
The  beach  is  a  great  mill  in  which  rock  fragments  are  being 
ground  by  the  waves  and  removed  toward  the  sea  (Fig.  203). 
Sometimes   these  beaches  are  of  great  extent ;   but  almost 


Boulders  worn  from  a  headland  by  ocean 
waves. 


THE  COAST  LINE. 


337 


always  their  typical  form  is  that  of  a  part  of  a  circle,  the 
curve  usually  being  a  beautifully  swinging  curve ;  and  there 
is  a  rhythm  which  appears  to  bear  a  relation  to  wave  force 
and  direction,  and  sediment  supply. 

Effect  of  Plants.  —  It  is  difficult  to  estimate  the  impor- 
tance of  the  seaweeds  which  cling  to  the  rocky  coasts. 
They  form  an  elastic  mat  which  protects  the  rock  from 
the  beating  of   the  waves  (Fig.  204);  and  upon  their  own 


Fig.  203. 
A  rocky  beach  on  the  exposed  coast  of  Cape  Ann,  Mass. 

structure,  which  is  capable  of  being  replaced  if  damaged, 
they  receive  the  destructive  blows  of  the  waves.  Along 
the  rocky  coast  of  New  England,  the  seaweeds  cover  the 
rocks  from  near  the  line  of  mid-tide  to  a  depth  of  several 
feet  below  the  lowest  tide,  which  is  the  zone  Avhere  the 
waves  are  most  active.  If  it  were  not  for  this  covering, 
these  rocky  coasts  would  certainly  be  worn  back  with 
much  greater  rapidity  than  at  present. 

Another  way  in  which  plants  are  active  along  the  shore 
z 


338 


PHYSICAL   GEOGRAPHY. 


w^'sasmmimmmmnium 

W^!?'?'^m!^^'r«r^m^. 

^^^M 

Pl* 

^ 

p, 

(■ 

*V^^^j| 

w 

1  itr,    r  t^ttrnKi 

<^  1^^ 

" 

iB'iii 

^^^H^             ^'j(^.  ^T 

J^  '\   <-.«•■. 

.:^ 

«3»-... 

_..;.. 

HK 

^H&  ,.       -^^i'^  . , 

i^:*u.* 

F 

HI 

.,'*r^      -  jr^o*.j'jw!v' 

Hfe||Ji 

p&BwSSBI^- 

:^i'    -t#-^'^>^ 

*'*^^  i 

!^^^    -- 

^/^                IW                 "-^5^      , 

iWc,-  ' 

^ 

«,^|^ 

#1>       .'                               ..m^J.iSf 

.             / 

''^ 

te^^^^l^^^u. 

#  . 

• 

? 

.,jM^^i  _       •eN^ 

K^^ 

W 

^^ffllF 

pw^wPH^^s?^ 

'S^'SI^*  A. 

_^ 

^^^te^iU^HH^^BI^^^^BHHHHHi 

MtmMrnmtfwnsi 

SirjJ'WS^"'-. i"!*:  - ; 

■^-  -     -tW^-^S-^B.     Arf^hi 

^gm 

■  ■«i!«%ssr*,„,.i*»  — 

Fig.  204. 
Mat  of  seaweed  between  tides,  Cape  Ann,  Mass. 

line,  is  in  the  actual  construction  of  land.  On  the  Florida 
coast  there  is  a  peculiar  type  of  tree,  the  mangrove,  which 
has  the  remarkable  habit  of  growing  with  its  roots  in  salt 


Fig.  205. 
A  mangrove  swamp. 


THE  COAST  LINE. 


339 


water.  The  roots  extend  into  the  sea  in  a  network,  raising 
the  tree  trunk  above  the  sea  level,  as  if  it  were  on  stilts  (Fig. 
205)  ;  and  these  root-like  branches  of  the  tree  encroach  upon 
the  sea.  By  this  growth  of  the  mangrove,  seacoast  swamps 
are  produced  in  the  shallow  waters  near  the  tropics,  and 
in  this  way  the  coast  line  is  built  outward.     As  the  trees  die, 


Fig.  206. 
A  salt  marsh  partly  filling  an  estuary,  Cape  Ann,  Mass. 


their  fragments  accumulate  in  the  shalloAv  water ;  and  be- 
tween the  roots  sediment  is  entangled,  so  that  little  by  little 
the  land  is  actually  built  up  and  the  salt  water  displaced  by 
swamp. 

Even  more  important  than  the  mangrove,  is  the  action  of 
some  of  the  grasses  which  grow  in  the  shallow  water  of  pro- 


340  PHYSICAL   GEOGRAPHY, 

tected  bays  and  estuaries  (Fig.  206).  These  salt  marsh  and 
eel  grasses  are  able  to  live  where  the  waves  are  not  too 
violent;  and  by  their  growth  and  death,  as  well  as  by  their 
action  in  entangling  and  causing  the  deposit  of  sediment, 
they  are  important  aids  in  the  lilling  of  these  shallow 
bays.  Along  the  eastern  coast  of  the  United  States  there 
are  thousands  of  square  miles  of  salt  marsh  which  are  m 
large  part  the  result  of  this  action  of  vegetation.  The  m  .^rsh 
is  built  up  to  a  level  just  above  that  of  the  highest  tide ;  and 
along  the  coast  of  this  region,  there  is  every  gradation  be- 
tween dry  land  and  the  shallow  water  of  enclosed  bays,  upon 
which  the  marine  vegetation  is  just  beginning  to  encroach. 
One  sees  it  in  almost  every  bay  and  estuary,  from  the  Caro- 
linas  northward  to  the  boundary  of  the  country.  There  are 
vast  areas  of  this  salt  marsh  in  the  lagoons  behind  the  bars 
which  are  formed  along  the  southern  coast. 

Effect  of  Animals.  —  There  are  many  ways  in  which  animals 
are  changing  the  form  of  the  coast  line,  by  far  the  most  im- 
portant being  the  action  of  coral  animals.  These  creatures 
are  able  to  live  only  under  certain  very  favorable  conditions. 
The  water  must  be  warm,  and  the  temperature  must  always 
remain  above  68°.  It  must  also  be  clear  and  free  from  sedi- 
ment. The  animals  cannot  live  in  depths  greater  than  100 
feet,  nor  can  they  thrive  unless  there  is  a  free  exposure  to 
currents  and  waves,  which  bring  food  to  them.  Therefore 
we  do  not  find  that  the  coasts  of  the  tropical  regions  are 
always  made  by  corals. 

Where  conditions  are  favorable,  corals  thrive  in  a  marvel- 
ous manner  (Figs.  79  and  207).  They  live  in  an  abundance 
that  is  hardly  equaled  by  any  of  the  other  marine  animals. 
Each  individual  builds  a  skeleton  of  carbonate  of  lime,  and 
these,  combining,  form  a  coral  mass,  which  upon  the  death  of 
the  animals,  is  left  behind  to  enter  into  the  formation  of  a 


THE  COAST  LINE. 


341 


limestone  rock.  The  corals  grow  along  the  coast,  forming 
large  reefs ;  and  at  times  they  produce  reefs  at  a  considerable 
distance  from  the  shore,  which  are  then  known  as  harrier 
reefs.  The  Great 
Barrier  Reef  of 
Australia  ex- 
tends along  the 
coast,  with  some 
interruption,  for 
a  distance  of 
1000  miles ;  and 
at  times  its  dis- 
tance is  60  miles 
from  the  shore, 
being  the  most 
extensive  growth 
of  coral  in  any 
single  region  in 
the   world.     Its 

width  at  the  surface  is  rarely  more  than  one  or  two  miles. 

At  times  the  coral  builds  isolated  islets,  which  are  often 

known  as  kei/s,  and  which  are  so  well  illustrated  by  the  keys 


Fig.  207. 
A  coral  reef  on  the  Australian  coast. 


SCALE  OF  MILES 

....••;;;•••••...•;•■        ^ 

K                                                    .•'        .•■*           •.-.'         ^       ;  •.     / 

/         ..■:.  .•-•••■■>   ■^••■>...%: 

^y                                   .•      -■•         COTTRELL  KEY            -•..    '*'•••.       .. 
^                            .■■•     MULE  KEY  9                o  MULLET  KEY          /  :..\ 

0      1       2 

3     4      5 

/marquesasI3 

^'      KEYS         ^ 

S                         /    "■■'      %                    SNIPE  KEY  OR  Aim.           *..■■■■'■;..■>■ 
0                       .'        '"■•■._                                                                     ■■■'    \ 

1           \,.i;-:::y        keyc^-«^"      /" 

"^          BOCA  GRANDE%>                  WOMAN  KEY^                       ,...--" 
KEY       ■•,     ^ .^^                  .3: ..•••• 

mankeV ■" 

It.D.SttvoiM,  ir.T. 

Fig.  208. 

Atoll-like  keys  on  the  Florida  coast. 

842 


PHYSICAL   GEOGRAPHY. 


at  the  southern  end  of  Florida  (Fig.  208).  In  the  mid- 
ocean,  particularly  in  the  South  Pacific,  the  coral  growth 
forms  ring-like  islands,  which  are  known  as  atolls  (Fig.  209). 
Sometimes  these  are  nearly  perfect  rings,  enclosing  an  area 
of  water  which  is  connected  with  the  sea  by  a  small  opening. 
The  atoll  rises  above  the  level  of  the  sea  to  a  height  suffi- 
cient for  the  growth  of  trees,  and  many  of  these  islands  are 
inhabited  by  man.  The  reason  for  their  elevation  above  the 
sea  is  the  washing  action  of  the  waves,  combined  with  the 


Fig.  209. 
An  atoll  in  the  South  Pacific. 


blowing  of  the  wind,  which  drifts  the  coral  sand  into  mounds. 
On  all  of  these  reefs,  corals  are  still  living  and  growing  where 
exposed  to  the  action  of  the  waves  and  currents  which  are 
bringing  food  to  them.  It  is  found  that  the  coral  reefs  are 
better  developed  on  coasts  which  are  exposed  to  the  oceanic 
currents  of  tropical  origin.  As  is  so  well  illustrated  in  the 
Bermuda  Islands,  which  are  in  latitude  32°  N.,  corals  mav  be 
developed  where  these  currents  extend  their  warmth  into 
latitudes  well  beyond  the  tropics. 


THE  COAST  LINE. 


343 


The  cause  for  atolls  is  at  present  in  dispute,  and  it  does 
not  seem  desirable  to  consider  the  question  as  to  which  ex- 
planation is  correct.  The  one  which  has  been  before  us  for 
the  longest  time  (having  been  proposed  by  Darwin),  and  is 
accepted  by  many  geologists,  is  that  the  atolls  are  nothing 
more  than  reefs  which  once  surrounded  volcanoes  that  have 
since  disappeared  by  submergence  (Fig.  210).  As  the  cone 
sank  beneath  the 
water,  the  corals 
built  the  reef  higher 
and  higher,  so  that 
even  after  the  cone 
had  entirely  disap- 
peared, its  position 
was  indicated  by  the 
ring-like  reef.  Cer- 
tainly this  seems  to 
be  a  true  explanation 
for  some  atoll  reefs ;  but  for  others,  another  explanation  is 
very  likely  necessary.  The  great  barrier  and  fringing  reefs 
are  merely  formed  by  the  growth  of  the  coral  along  or  near 
the  coast  line. 

Changes  in  Coast  Form.  —  With  change  in  the  conditions, 
a  coast  may  assume  entirely  different  characteristics.  If,  for 
instance,  the  clear  waters  of  some  coral  coasts  are  for  any 
reason  changed  to  muddy  water,  coral  life  is  driven  out,  and 
the  muddy  or  sandy  shore  takes  its  place.  If  land  is  ele- 
vated, or  if  it  is  depressed,  the  form  of  the  coast  is  very 
greatly  changed.  The  agents  of  denudation  are  always  at 
work  tending  to  alter  the  coast  form.  Therefore  the  shore 
line  which  we  know  at  present,  is  merely  a  temporary  fea- 
ture, merely  the  stage  Avhich  has  been  reached  at  the  present 
time;  and  it  is  far  from  the  condition  which  has  existed  in 


Fig.  210. 

Diagram  to  illustrate  one  explanation  of  the 
origin  of  atolls.  V,  volcanic  cone;  art,  bb, 
cc,  successive  levels  of  the  sea ;  dd,  ee,  and  ii 
showing  corresponding  condition  of  the  reef, 
finally  producing  the  atoll  ii  when  the  volcano 
was  entirely  submerged. 


344  PHYSICAL   GEOGBAPHY, 

the  past,  and  probably  from  the  condition  which  will  exist 
in  the  future.  In  imagination  we  are  able  to  look  back  to 
the  time  when  the  eastern  coast  of  the  United  States  had 
not  its  present  irregularity;  and  by  geological  evidence  we 
are  also  certain  that  but  a  short  time  ago  Florida  was 
not  present  as  a  peninsula.  The  delta  of  the  Mississippi  is 
a  growth  of  very  recent  date  ;  and  preceding  its  formation 
an  estuary  extended  up  the  valley  of  the  Mississippi,  at  least 
as  far  as  Arkansas. 

Our  knowledge  of  the  geology  of  the  coast  line  is  not  suf- 
ficiently detailed  to  allow  us  to  study  all  the  changes  that 
are  going  on ;  but  any  one  who  dwells  by  the  coast,  will  be 
able  to  see  that  there  are  some  changes  now  in  progress.  A 
visit  to  the  seacoast  in  time  of  storm,  or  indeed  to  the  lake 
shore,  will  convince  any  one  that  there  are  changes  in 
progress,  which,  as  a  result  of  the  repetition  of  this  action 
through  scores  of  years,  must  produce  perceptible  changes. 

Islands.  —  There  is  a  very  great  variation  in  the  size  of 
oceanic  islands,  in  the  distance  from  the  shore,  in  the  form, 
and  in  origin.  It  is  quite  customary  to  speak  of  two  classes 
of  islands  :  oceanic  and  continental,  the  oceanic  being  those 
which  occur  far  from  the  land.  These  oceanic  islands  are 
generally  of  three  classes :  (1)  those  that  are  formed  by 
volcanoes,  (2)  those  that  are  produced  by  the  folding  of 
mountains,  and  (3)  the  mid-ocean  coral  reefs.  Generally 
they  are  small,  and  they  often  occur  in  chains,  as  if  they 
represented  tops  of  mountain  peaks  along  some  ridge  that  is 
partly  beneath  the  ocean.  In  some  instances,  soundings  have 
shown  that  this  is  actually  the  case. 

Near  the  coasts  of  continents  we  have  the  same  kinds  of 
islands.  The  Japanese  archipelago  is  apparently  a  moun- 
tain chain  which  is  now  in  process  of  being  formed.  In  the 
Mediterranean,  among  the  West  Indies,  and  elsewhere,  there 


THE  COAST  LINE, 


345 


are  many  instances  of  volcanic  peaks  Avhich  form  islands  not 
far  from  the  coast ;  but  probably  nine  out  of  ten  of  the 
islands  of  the  world  have  resulted  from  causes  other  than 
these.  Most  of  the  islands  are  derived  either  from  the 
submergence  of  the  land  (Fig.  211),  or  from  the  building  up 


j^ 


,'  h 


,  ,  FL'/rNGi'PT.  Mare 

i^r^        Pi.  ^         ^  ^ 

Staple's  Pt.  ^     a 

■^^LANE'S  I. 


UPPER/rl 

GOOSe/lj/ 

BIBBER'S  I.  ^  t>^^ 

<s^  y-%  /J?*^"       /A. 

^EAT  ^SEI  A. 

MOSHIER'S  I.       i'  'V        -A  '*' 


'yjy.,^.-^-    y^/M'P     '"1."''?=^ 


.'^ 


ITTLE  BANGS /I.  r  Harbor 


HORSE  I. 


-.    ly      ^      (J  ^    ^    </  .«?  ^  ?""■■' 


9 

FLAG  1.^ 


^^ 


/JEWELL'S  I. 


HASKELL  I. 


N. 


r\RAGGED  I. 


WOOD  I 


Bald  Head  <~^^ 


SCALE  OF  MILES 


1 


5         B.D.Strratt.N.Y. 


Fig.  211. 
The  islands,  capes,  and  promontories  of  Casco  Bay,  Me. 

of  the  coast  line  by  wave  action  (Fig.  194).  As  has  been 
stated,  the  waves  throw  up  bars  in  favorable  places,  the 
wind  gathers  the  sand  and  blows  it  into  sand  dunes,  and 
islands  of  this  origin  are  found  in  abundance  along  many 
coasts.  South  of  Cape  Hatteras  there  is  a  line  of  such 
islands,  which  are  due  partly  to  wave  and  partly  to  wind 


346  PHYSICAL   GEOGRAPHY. 

action,  and  which  stretch   along  the  coast  parallel    to   the 
mainland,  and  but  a  short  distance  from  it. 

When  an  irregular  coast  is  lowered,  the  sea  rises  around 
the  hills,  forming  islands.  This  is  excellently  illustrated 
on  the  coast  of  Maine  (Fig.  211),  where  the  thousands  of 
islands  and  islets  are  in  most  instances  the  direct  result  of 
the  partial  lowering  of  hilly  land  beneath  the  sea  level. 
There  are  some  minor  ways  in  which  islands  are  produced, 
but  these  are  by  far  the  most  important. 

Being  surrounded  entirely  by  water,  islands  are  peculiarly 
liable  to  destruction  by  wave  action  (Fig.  195).  They  are 
open  to  attack  from  every  side,  and  if  they  happen  to  be  in 
the  ocean  far  from  the  land,  they  may  be  very  rapidly  de- 
stroyed. Even  extensive  volcanoes  in  the  mid-ocean  are 
quickly  worn  away  as  soon  as  the  volcanic  fires  have  ceased 
to  add  material  in  place  of  that  which  the  sea  removes. 
Among  the  Hawaiian  Islands,  the  smaller  islands,  which  were 
formed  by  volcanoes  now  extinct,  are  rapidly  being  destroyed; 
and  the  same  is  noticed  throughout  the  Pacific,  as  well  as 
among  the  volcanic  islands  of  the  Atlantic  (Fig.  125).  A 
volcano  formed  in  the  Mediterranean,  in  1831,  and  known  as 
Graham's  Island,  reached  a  height  of  200  feet,  with  a  circum- 
ference of  three  miles ;  but  in  the  course  of  a  few  years  it 
was  entirely  destroyed  by  wave  action. 

Promontories.  —  Capes  and  promontories  belong  to  the 
same  class  of  seashore  forms,  the  difference  being  merely  in 
size.  They  are  produced  in  several  ways.  Some  of  the 
largest  promontories  are  parts  of  mountain  folds  in  the  sea. 
Others  are  areas  of  hard  rock  which  have  resisted  the  agents 
of  denudation  and  formed  highlands,  which,  when  a  sub- 
mergence of  the  land  took  place,  remained  above  sea  level, 
while  the  lower,  neighboring  parts  of  the  land  were  sub- 
merged.    Labrador  and  Nova  Scotia  illustrate  this. 


THE  COAST  LINE. 


347 


By  far  the  greater  number  of  capes  are  a  result  of  this 
kind  of  submergence  of  land  (Fig.  211).  The  peninsula 
of  Florida  owes  its  existence,  in  part  at  least,  to  the  action 
of  corals,  which  have  built  the  southern  half.  The  warm 
Gulf  Stream,  which  bathes  this  coast,  has  brought  food  to 
the  coral  animals,  and  these  have  built  reefs.  In  the  vicinity 
of  Key  West  the  peninsula  of  Florida  is  still  growing  south- 


FiG.  212. 
A  bluff  cut  in  clay,  on  the  Lake  Michigan  shore. 

ward,  the  keys  being  merely  small  parts  of  a  submarine 
plateau  which  are  above  the  sea.  Some  small  capes  are 
caused  by  the  building  out  of  the  land  through  the  deposit 
of  sediment.  Sandy  Hook  is  an  illustration  of  this,  and 
many  of  the  hooks  and  spits  of  sandy  coasts  are  of  the  same 
origin  (Figs.  196  and  197). 

Lake  Shores.  —  On  the  shores  of  lakes  we  have  instances 
of  changes  due  to  wave  action,  of  cliffs  formed  by  the  under- 


348 


PHYSICAL   GEOGRAPHY. 


cutting  of  waves  (Fig.  212),  of  the  building  of  bars  and 
hooks  (Fig.  197),  of  the  formation  of  beaches  (Fig.  213), 
and  indeed  of  nearly  all  the  phenomena  of  the  seashore. 
Since  many  lakes  are  nothing  but  river  valleys  that  have 
been  dammed  through  some  agency,  both  islands  and  capes 
are  often  produced.  These  occur  where  the  necessary 
irregularities  existed  on  the  side  of  the  valley  which  has 

been  filled  Avith 
water.  Where 
there  were  trib- 
utaries to  the 
stream  which  has 
been  transform- 
ed to  a  lake,  the 
lake  water  enters 
in  the  form  of  a 
bay ;  and  the 
hillside  border- 
ing this  bay  ex- 
tends into  the 
lake  as  a  cape.  Sometimes  there  are  hundreds  of  islands 
in  the  lake  waters.  The  Thousand  Islands,  at  the  outlet  of 
Ontario,  are  the  tops  of  low  hills,  the  sides  of  which  are 
submerged  beneath  the  lake  waters. 

In  the  lakes  there  is  usually  less  violent  action  than  on  the 
seashore,  partly  because  the  waves  do  not  rise  to  the  height 
of  the  true  ocean  wave,  and  partly  because  the  tide  action  is 
absent.  The  shores  of  most  of  the  smaller  lakes  bear  a  closer 
resemblance  to  those  of  the  partly  enclosed  harbors  and  bays 
of  the  seacoast,  than  they  do  to  the  exposed  ocean  coasts. 
But  except  in  intensity  of  development,  there  is  little  differ- 
ence between  lake  shores  and  seacoasts. 

Fossil  Shore  Lines.  —  Coast  lines  are  sometimes  abandoned, 


Fig.  213. 

A  lagoon  enclosed  behind  a  beach  barrier  on  the  shore 
of  Lake  Michigan. 


THE  COAST  LINE.  349 

and  may  then  be  found  on  the  dry  land.  This  happens 
when  the  land  on  the  seashore  rises,  or  when  for  any 
reason  a  lake  disappears.  These  shore  lines  have  all  the 
features  of  those  now  forming  in  the  sea  or  lake.  There  are 
fossil  beaches  (Fig.  170),  bars,  spits,  hooks,  wave-cut  cliffs, 
etc. ;  and  immediately  after  their  abandonment  by  the  waves 
their  features  are  very  distinct ;  but  in  a  short  time  they 
begin  to  crumble  away  under  the  action  of  denudation,  and 
before  long  no  sign  is  left  to  tell  of  the  change.  Such 
ancient  shore  lines  on  the  coast  of  New  England  and 
Labrador,  tell  of  a  recent  submergence  of  the  land  ;  and 
the  shore  lines  south  of  the  Great  Lakes,  tell  us  that  they 
once  covered  a  considerable  area  of  country  south  of  their 
present  site. 


-•o*- 


REFERENCE    BOOKS. 

Much  of  interest  is  found  in  Gilbert's  "Lake  Bonneville,"  referred  to  at  the 
end  of  Chapter  XVI.  A  special  paper  on  shore  lines,  by  the  same  author,  is 
found  in  the  Fifth  Annual  Report  U.  S.  Geological  Survey,  Washington, 
1885. 

Shaler.  —  Sea  AXD  Laxd.  Scribner,  New  York,  1894.  8vo.  $2.50.  (This 
contains  much  of  value  upon  shore  lines,  written  in  Professor  Shaler's 
remarkably  entertaining  style.) 

Dana.  —  Corals  and  Coral  Islands.  Dodd,  Mead  &  Co.,  New  York,  1890. 
8vo.     $5.00. 

Darwin. — The  Structure  and  Distribution  of  Coral  Reefs.  Smith, 
Elder  &  Co.,  London  (Appleton  &  Co.,  New  York,  Agents).  Third  Edi- 
tion, 1889.     12mo.     $2.00. 

For  Harbors,  see  Shaler,  Thirteenth  Annual  Report  U.  S.  Geological 
Survey,  Washington,  1893. 

For  Salt  Marshes,  see  Shaler,  Sixth  Annual  Report  of  the  same,  1885. 


CHAPTER   XIX. 


PLATEAUS   AND   MOUNTAINS. 


Fig.  214. 
Pecos  River  valley,  southern  New  Mexico. 


Plateaus.  — A  plateau  is  a  level-topped  area  at  a  consider- 
able elevation  above  the  sea.  In  many  respects  it  resembles 
I-  a  plain  (Figs.  214  and 

215),butusually  it  is  not 
so  level;  and  the  ordi- 
nary distinction  between 
plains  and  plateaus  is 
based  upon  elevation. 
Both  are  relatively  level 
areas  ;  and  both  plains 
and  plateaus  are  usually 
composed  of  sedimentary  rocks  in  a  nearly  horizontal  position. 
Plateaus   are   gen-     t~   r.,,,,.^.™ ,    _.        _,. 

erally  associated  with 
mountains,  and  most 
mountains  rise  above 
a  basal  platform 
which  is  a  true  pla- 
teau. Thus  at  the 
eastern  base  of  the 
Rocky  Mountains, 
the  plateau  of  the 
Mississippi  valley 
ascends  to  the  very 
mountain  base,  while  on  the  western  side  there  is  the  ex- 
tensive  interior   plateau   of   the    Great   Basin.      In   nearly 

350 


-jwir-v4w»  •  «*« 


Fig.  215. 
Plain  in  the  valley  of  the  Red  River  of  the  North. 


PLATEAUS  AND  MOUNTAINS. 


351 


every  continent  there  are  large  plateau  areas,  but  nowhere 
is  this  form  of  topography  better  developed  than  in  the 
central  part  of  Asia,  north  of  the  Himalayas,  where  for 
thousands  of  square  miles  the  plateau  rises  to  an  eleva- 
tion of  several  thousand  feet.  A  portion  of  the  Indian 
plateau  region,  as  well  as  a  part  of  the  plateau  of  the  Rocky 
Mountain  area,  is  covered  with  an  extensive  series  of  lava 
flows  which  have  been  sent  to  the  surface  through  great  fis- 
sures. The  lava-capped  plateau  of  the  Deccan  has  an  area 
of  200,000  square  miles, 
and  that  of  the  Snake 
River  valley  of  Idaho  also 
covers  an  immense  area, 
with  a  depth  in  places 
greater  than  3000  feet. 

Since  they  are  in  associa- 
tion with  mountains,  these 
plateaus  are  very  liable 
to  be  arid.  Many  of  the 
interior  deserts  between 
mountain  ranges  are  real 
plateaus,  as  is  so  Avell  illus- 
trated in  the  western  part  of  our  own  country.  Among  the 
various  mountains  of  the  western  half,  of  the  continent,  the 
prevailing  condition  is  that  of  arid  plateaus  broken  by  occa- 
sional mountain  ranges ;  and  the  conditions  of  dryness  and 
absence  of  forest-covering,  exist  also  on  the  plateau  east  of 
the  Rocky  Mountains.  This  plateau  region  is  usually  spoken 
of  as  the  Plains  of  the  Far  West.  In  the  general  levelness 
of  the  surface,  and  also  in  the  absence  of  forest,  it  resembles 
the  prairies  east  of  the  Mississippi.  But  in  the  case  of  the 
prairie,  the  forest  is  not  absent  because  of  dryness,  while  this 
is  the  cause  for  its  absence  in  the  so-called  Plains. 


Fig.  216. 

Taos  Mountains,  New  Mexico,  rising  above 

an  extensive  plateau. 


352  PHYSICAL   GEOGBAPHY. 

Since  plateaus  are  elevated  above  the  general  level  of  the 
country,  they  are  often  very  deeply  carved  by  river  erosion. 
Some  of  the  most  remarkable  cases  of  deep,  narrow  river 
valleys  are  found  among  high  plateaus.  Nowhere  is  this 
better  illustrated  than  in  the  high  plateau  of  Utah  and 
Arizona,  through  which  is  cut  the  remarkable  canon  of  the 
Colorado  (Fig.  142  and  Plate  28).  In  this  respect  also, 
there  is  a  difference  between  plateaus  and  low  plains;  for 


-i„  !•>*.  3  -rs 


r   .1 


Fig.  217. 
The  plateau  near  the  Colorado  River. 


the   latter   are  not  crossed   by  deep  valleys,  because   their 
surface  is  usually  not  far  above  the  level  of  the  sea. 

As  is  so  strikingly  shown  in  the  cailon  of  the  Colorado,  the 
ruggedness  of  the  river  valley  formed  among  plateaus,  de- 
pends in  no  small  degree  upon  the  climatic  conditions.  The 
climate  is  so  dry  that  the  agents  of  weathering  are  not  very 
important ;  and  consequently  the  main  work  of  sculpturing  is 
done  by  the  river  itself.  This  causes  a  deep,  angular  trench, 
whose  angularity  is  preserved  because  the  rocks  which  border 


PLATEAUS  AND  MOUNTAINS. 


353 


'"V- ^Vl  J*?£*#'3?i'W>>?^»5^S.^J 


the  valley  are  not  raj)idly  melted  away  under  the  action  of 
rain  and  frost.  As  a  result  of  this  peculiarity,  the  charac- 
teristic topography  of  the  plateau  in  an  arid  region,  is  that 
of  occasional  level  stretches  with  steeply  sloping  boundaries 
(Figs.  142  and  217).  The  country  is  often  cut  into  a  series 
of  terraces,  one  step  above  and  beyond  another.  In  the 
western  part  of  this  country,  the  level-topped  sections  of  the 
plateau  have  been  given  the  name  of  mesa^  which  means 
table  ;  and  when  the  level-topped  sections  are  small,  they  are 
called  huttes  (Figs.  218  and  257). 

Mountains :  Characteris- 
tics of  Mountains.  —  Popu- 
larly considered,  a  moun- 
tain is  any  unusual 
elevation ;  and  upon  the 
plains  of  Texas,  an  eleva- 
tion of  100  or  200  feet 
passes  as  a  mountain,  while 
in  a  thoroughly  mountain- 
ous district,  elevations  of 
1000  or  2000  feet  are  known 
as  hills.  We  shall  accept 
this  common  usage  of  the 
term  mountains ;  but  it  will  be  pointed  out  that  there  are 
various  kinds,  derived  in  a  variety  of  ways.  By  far  the 
greater  number  of  mountains,  and  certainly  the  most  pro- 
nounced in  the  world,  are  the  direct  result  of  folding  of 
the  earth's  crust,  in  nearly  all  cases  combined  with  a  great 
amount  of  destructive  action  of  denudation.  Along  certain 
lines,  the  rocks  of  the  crust  are  folded  and  broken  into  great 
ridges  and  chains,  which  in  some  cases  extend  from  one  end 
of  a  continent  to  another.  Indeed,  in  the  American  conti- 
nents there  is  practically  one  continuous  set  of  rock  folds, 

2a 


Fig.  218. 
Butte  in  New  Mexico. 


354 


PHYSICAL   GEOGRAPHY, 


from  the  southern  end  of  South  America  to  the  northern 
part  of  Alaska. 

A  set  of  rock  folds  forming  a  great  mountain  series  is 
generally  known  as  a  system.  The  Rockies  form  a  system 
of  mountains,  and  several  systems  combined  form  a  cordillera 
(Fig.  129).  This  is  illustrated  in  the  western  part  of  this 
country,  which  is  crossed  not  only  by  the  Rocky  Mountains, 


Fig.  219. 
A  talus  slope  at  the  base  of  a  mountain  ridge.     (Elk  Mountains,  Colorado.) 


but  by  the  Basin  Ranges,  the  Sierra  Nevadas,  and  the  Coast 
Ranges.  When  we  examine  any  single  mountain  system, 
as,  for  instance,  that  of  the  Rockies,  we  find  it  to  be  com- 
posed of  various  parts.  There  are  individual  ranges  among 
these  mountains,  and  any  single  range  is  also  found  to  be 
composed  of  separate  parts,  to  which  the  name  ridges  (Fig. 
219)  may  be  given.     In  all  of  these  cases,  the  striking  pecui- 


PLATEAUS  AND  MOUNTAINS. 


355 


iarity  is  that  the  length  of  the  mountains  greatly  exceeds 
both  the  width  and  the  height.  The  cordillera  and  the 
system  may  extend  for  a  thousand  or  more  miles ;  the 
range  may  extend  for  a  distance  of  more  than  a  hundred 
miles ;  but  the  ridge  is  usually  only  a  few  miles,  or  at  most 
a  few  score  of  miles,  in  extent. 

There  are  prominent  parts  of  mountains  which  do  not 
have  this  charac- 
teristic of  the 
ridge,  and  these 
are  spoken  of  as 
peaks  (Fig.  220). 
In  nearly  all  cases 
the  real  mountain 
peak  is  merely  a 
portion  of  a  ridge 
or  chain,  which 
for  some  reason 
stands  up  higher 
than  the  sur- 
rounding parts. 
The  usual  cause 
for  this  greater 
elevation  of  one 
portion,  is  the 
presence  of  some  hard  rock  which  resists  weathering.  While 
mountains  are  forming,  and  after  they  have  been  formed, 
they  are  subjected  to  the  agents  of  denudation,  which  tend 
to  wear  them  away;  and  in  this  process  of  destruction,  the 
harder  rocks  are  left  higher  than  the  softer  ones.  In  look- 
ing among  the  more  pronounced  mountain  peaks  of  the 
world,  we  find  that  in  most  cases  these  are  made  of  some 
particularly  durable  rock.     Pike's  Peak  is  made  of  granite ; 


Fig.  220. 
Matterhorn,  a  Swiss  mountain  peak. 


356  PHYSICAL   GEOGRAPHY, 

the  Matterhorn  (Fig.  220)  of  the  Alps  is  composed  of  a 
similar  hard  crystalline  rock,  and  the  White  Mountains  of 
New  Hampshire,  the  peaks  of  the  Adirondacks,  etc.,  have 
the  same  characteristic.  This  is  the  typical  mountain  peak, 
a  form  resulting  partly  from  the  folding  of  the  rocks  during 
the  formation  of  the  mountains,  partly  from  the  differences 
in  the  hardness  of  rock,  and  partly  from  denudation.  In 
the  longitudinal  parts  of  mountains,  the  fold  is  the  most 
prominent  factor ;  in  these  more  nearly  circular  portions, 
the  factor  of  prominence  is  rather  that  of  denudation. 

There  are  other  forms  of  mountain  peaks  in  which  rock 
folding  does  not  enter  as  a  prominent  cause.  The  most 
abundant  of  these  are  the  volcanic  peaks  whose  origin  and 
characteristics  are  discussed  in  the  next  chapter.  In  many 
parts  of  the  world,  particularly  on  plateaus,  there  is  a  form 
of  elevation  often  called  a  mountain,  which  is  the  result 
merely  of  denudation  acting  upon  strata  whose  position  is 
nearly  horizontal.  There  has  been  no  folding,  and  no  dis- 
turbance of  the  rocks  other  than  that  of  elevation  ;  but  hills 
or  peaks  have  been  cut  out  by  erosion,  and  these  now  stand 
above  the  general  level  of  the  country.  In  the  western 
part  of  the  United  States  they  are  often  knoAvn  as  buttes. 
By  some  they  are  called  hills  of  circumdenudation,  because 
all  around  the  elevated  portion  the  rocks  have  been  cut  away 
(Figs.  218  and  257). 

Next  in  prominence  to  the  elevations  of  the  mountains  are 
the  depressions.  Between  the  ridges,  systems,  and  peaks, 
there  are  valleys ;  and  these  have  quite  distinct  character- 
istics. Between  systems,  and  really  forming  a  natural  part 
of  Cordilleras,  there  are  often  great  valleys,  sometimes  hun- 
dreds of  miles  in  width  and  length,  to  which  the  name  inte- 
rior basin  is  generally  given.  They  are  great  plateau  areas 
between  mountain  walls,  and  they  are  usually  more  or  less 


PLATEAUS  AND  MOUNTAINS.  357 

broken  by  mountain  ridges.  Sometimes,  in  part  of  tbeir 
area,  there  is  drainage  to  the  sea  ;  but  very  often,  and  as 
a  characteristic  feature,  a  part  of  the  drainage  finds  its  way 
into  these  great  troughs  and  does  not  escape  to  the  sea,  but 
is  returned  to  the  air  by  evaporation. 

The  Great  Basin  of  the  United  States  has  an  area  of  over 
200,000  square  miles;   but  notwithstanding  the  great  size  of 


Fig.  221. 
A  mountain  park  (Baker's) . 

the  basins  of  interior  drainage  on  this  continent,  these  form 
but  3.2  percent  of  the  total  continental  area.  In  Australia 
nearly  52  per  cent  of  the  area  is  in  the  condition  of  interior 
drainage,  while  31  per  cent  of  Africa  is  in  the  same  con- 
dition, and  28  per  cent  of  the  continental  mass  of  Eurasia 
is  an  enclosed  basin.  The  Sahara  interior  basin  is  16  times 
as  large  as  our  Great  Basin,  and  the  interior  basin  region  of 
Asia  occupies  an  area  23  times  as  great  as  that  of  the  west. 


358 


PHYSICAL   GEOGBAPHY. 


Between  mountain  ridges  and  chains,  there  are  often 
longitudinal  valleys  of  considerable  size,  extending  par- 
allel to  the  chains  between  which  they  occur.  These  are 
among  the  striking  features  of  mountains,  and  they  are 
generally  occupied  by  streams  which  are  evidently  too  small 
to  have  carved  such  immense  valleys.  When  the  rock 
structure  is  studied,  it  is  evident  that  these  valleys  repre- 
sent either  down-folded  portions  of  the  crust,  or  else  portions 


Fig.  222. 
A  mountain  gorge  in  the  high  Andes  of  Peru. 

that  have  been  broken  or  faulted  down.  Where  these  val- 
leys occur  between  peaks  and  ridges,  forming  amphitheaters 
among  the  mountains,  they  produce  a  characteristic  valley, 
which  among  the  Rocky  Mountains  is  given  the  name  of 
park  (Fig.  221). 

Occasionally  the  streams  have  carved  mountain  gorges, 
and  even  in  some  cases  have  cut  entirely  across  the  ridges, 
forming  valleys  which  are  characterized  by  remarkably  steep- 
sided  gorges  (Fig.  144).     They  furnish  some  of  the  most 


PLATEAUS  AND  MOUNTAINS. 


359 


Striking  bits  of  mountain  scenery,  and  in  traveling  across  a 
mountain  ridge  upon  a  railroad,  one  is  often  carried  through 
these  gorges,  which  furnish  the  sole  means  of  easy  passage 
for  the  railroad  (Figs.  134  and  222).  Low  points  in  moun- 
tain ridges  are  known  as  passes.  Sometimes  these  are 
merely  parts  of  the  mountain  which  were  not  folded  so 
high  as  other  portions;  but  in  many  cases  they  are  valleys 


Fig.  223. 
Mount  of  the  Holy  Cross,  Colorado  —  above  the  timber  line. 

at  the  headwaters  of  streams.  Two  streams  head  together 
in  a  mountain  ridge,  and  these  lower  the  ridge  at  this 
point,  producing  a  gap,  which  is  usually  taken  advantage 
of  as  a  means  of  passage  across  the  mountains. 

Mountains  in  their  best  development  are  extraordinarily 
rugged.  They  rise  in  a  series  of  slopes,  sometimes  moder- 
ate, but  at  other  times  very  precipitous.     They  are  cut  by 


860  PHYSICAL   GEOGRAPHY. 

valleys  which  are  often  bounded  by  true  precipices.  The 
hard  rocks  stand  up  precipitously,  while  the  softer  strata 
furnish  more  gentle  slopes.  The  mountain  form,  in  all  of 
its  irregularity  and  variety,  depends  upon  the  action  of  the 
agents  of  denudation  upon  the  rocks  of  different  hardness 
which  have  been  folded  into  more  or  less  complex  attitudes. 
Generally  the  mountains  are  regions  of  heavy  rainfall ;  but 
if  they  rise  to  a  very  considerable  elevation,  this  comes 
mostly  in  the  form  of  snow ;   and  even  within  the  tropics. 


Fig.  224. 
Trail  on  Long's  Peak,  Colorado. 

the  high  mountain  peaks  may  be  snow-capped  throughout 
the  year.     Near  the  base  of  the  -mountains,  the  fact  of  heavy 
rainfall  causes  the  growth  of  luxuriant  vegetation,  generally 
in  the  form  of  a  dense  forest  covering.     As  one  ascends  the  < 
mountain  sides  toward  the  upper  regions  of  cold,  the  forest 
gradually  changes  in  character,  at  first  assuming  the  habit 
of  the  northern  forest,  then  becoming  more  and  more  sparse  , 
(Fig.   221),  and  finally,  when  the  timber  line  is  reached,/ 
entirely  disappearing  (Figs.  QQ  and  223).      At  the  timber \ 


PLATEAUS  AND  FOUNTAINS. 


361 


line  the  forest  is  replaced  by  scattered  patches  of  trees ;  and 
above  this,  these  forms  of  vegetation  disappear. 

As  these  upper  regions  of  the  mountains  are  approached, 
the  peak  becomes  more  and  more  rugged.  Generally  the 
surface  of  the  ground  is  strewn  with  loose  boulders,  which 
have   been   broken   from   the   rock   that   formed   the   peak 


Fig.  225. 
Mountain  ridge  on  the  Canadian  Pacific. 

(Fig.  224).  They  have  been  removed  from  the  ledge  by 
the  action  of  frost,  and  are  being  disintegrated.  Upon  these 
mountain  peaks,  because  of  the  great  cold,  frost  action  is 
very  important.  By  removing  all  loose  particles,  the  violent 
winds  check  the  formation  of  soil,  and  the  excessive  slopes 
also  tend  to  prevent  this ;  for  every  drop  of  water  that  falls, 
passes  down  the  steep  incline,  carrying  along  all  small  frag- 


362  PHYSICAL   GEOGRAPHY. 

ments.     The  absence  of  plants  removes  a  protective  covering 
that  is  important  in  modifying  the  action  of  weathering. 

The  form  and  ruggedness  of  the  mountain  chain,  ridge, 
or  peak  will  depend  upon  a  variety  of  circumstances,  chief 
among  which  are  the  kind  of  action  which  has  formed  the 
mountain,  the  position  and  structure  of  the  rocks  out  of 
which  the  mountain  is  made,  and  the  length  of  time  during 
which  denudation  has  been  acting  toward  the  destruction  of 
the  mountains.  Where  there  are  unusually  hard  layers  in 
a  mountain  ridge,  these  tend  to  remain  high  above  the  sur- 
rounding country,  and  the  mountain  always  has  the  ridge- 
like form  (Fig.  225)  ;  but  where  the  ridge  itself  has  been 
subjected  to  variations  in  folding,  in  the  course  of  time  its 
ridge-like  habit  may  be  destroyed.  The  massiveness  of  the 
rocks  forming  the  mountains  also  has  much  to  do  with  their 
ruggedness.  If  composed  of  a  series  of  strata  of  irregular 
hardness  (Figs.  261  and  262),  the  topography  will  be  very 
different  from  that  resulting  in  a  mountain  composed  of 
rocks  of  uniform  character  (Fig.  251).  The  most  precipi- 
tous and  rugged  of  mountains  are  those  made  out  of  rocks 
of  uniform  structure.  Some  of  the  ridges  in  the  Rockies 
are  made  of  massive  limestone,  and  among  these  there  are 
excessively  high  precipices. 

The  Origin  of  Mountains.  —  Several  theories  have  been  pro- 
posed to  account  for  the  formation  of  mountain  folds;  but 
at  the  present  time  no  one  of  these  can  be  said  to  be  thor- 
oughly satisfactory.  We  are  in  doubt  as  to  the  actual 
reason  for  the  folding  of  the  surface  rocks  along  certain 
lines.  This  much  is  quite  universally  agreed  upon, — that,  in 
one  way  or  another,  it  is  the  result  of  the  heated  condition 
of  the  interior  of  the  earth.  The  greater  number  of  geolo- 
gists also  believe  that  the  most  satisfactory  explanation  at 
present  before  us,  is  the  one  depending  upon  contraction. 


PLATEAUS  AND  MOUNTAINS.  S63 

The  interior  is  highly  heated,  and  this  heat  is  passing  from 
the  earth  into  space.  As  it  is  lost,  the  heated  interior  also 
necessarily  loses  bulk,  and  the  cold  solid  crust  attempts  to 
accommodate  itself  to  this  constantly  decreasing  interior. 
The  crust  itself  does  not  lose  in  bulk,  and  in  order  to  sur- 
round the  sphere,  which  is  constantly  having  its  diameter 
shortened,  it  must  wrinkle  ;  and  the  comparison  is  very  well 
made  between  this  supposed  action  of  the  crust  of  the  earth, 
and  that  which  happens  when  an  apple  is  dried  by  exposure 
to  the  air.  As  the  apple  dries,  water  passes  from  within,  and 
the  interior  portion  constantly  loses  in  size,  while  the  skin 
does  not  lose  bulk,  but  always  attempts  to  surround  the 
apple,  and  in  doing  so  produces  a  wrinkled  surface. 

The  mountain  and  continent  folds,  and  indeed  all  of  the 
expressions  of  frequent  movement  of  the  earth's  crust,  are 
believed  by  many  geologists  to  be  the  direct  result  of  this 
contraction  of  the  interior  ;  and  this  theory  for  the  forma- 
tion of  mountains  is  known  as  the  contraction  theory.  It  is 
possible  that  there  are  other  causes  aiding,  and  it  cannot  be 
denied  that  there  is  a  possibility  of  some  other  explanation. 
Our  knowledge  of  the  interior  of  the  earth  is  too  limited 
to  warrant  any  dogmatic  assertion  upon  hypothesis. 

The  growth  of  mountains  is  not  a  stupendous  overturning 
along  certain  lines,  but  rather  a  very  slow  upward  or  down- 
ward folding  of  a  portion  of  the  rocks.  From  all  the  evi- 
dence that  we  possess,  there  is  no  reason  for  believing  that 
any  mountain  chain  in  the  world  has  ever  grown  Avith  sud- 
denness. There  is  reason  for  believing  that  the  Coast 
Ranges  of  the  Pacific  slope  are  even  now  in  the  process 
of  growth,  and  this  is  certainly  true  of  the  Japanese  Islands 
and  of  the  Andes.  So  far  as  we  may  judge,  these  two  latter 
instances  are  illustrations  of  rather  rapid  mountain  growth; 
and  yet,  in  both  places,  people  find  it  possible  to  live  with  no 


364 


PHYSICAL   GEOGRAPHY. 


other  danger  than  that  coming  from  occasional  volcanic  erup- 
tions and  earthquake  shocks.  The  crust  of  the  earth  is 
not  convulsed,  but  is  folded  with  slowness.  This  is  true 
even  when  the  rocks  break  instead  of  bending.  Faults, 
representing  the  breaking  of  the  rocks  along  certain  planes, 
are  even  noAv  in  process  of  formation  in  various  parts  of  the 
world. 

If  we  examine  a  section  of  a  mountain,  we  find  the  rock 
strata  extending  from  the  earth  on  either  side  of  the  ridge 
(Fig.  226);  but  their  extension  into  the  air  has  been  pre- 
vented by  de- 
,^' ''    „         "'^v.  nudation.   The 

edges  of  the 
rock  layers 
have  been  trun- 
cated   by    this 

Fig.  226,  ^'^^'O"-      "   ^^e 

Section  across  a  mountain,  showing  normal  extension  of     continue        the 

^^^^*^*  strata  from  one 

side  to  the  other,  joining  like  layers  (Fig.  226),  we  find 
that  a  mountain  would  result  whose  height  would 
be  greater  than  anything  known  upon  the  surface  of 
the  earth.  Some  of  the  mountains  would  be  20,000  or 
30,000  feet  higher  than  at  present.  It  is  not  probable 
that  these  mountains  ever  did  extend  to  this  elevation ; 
but  rather,  that  as  the  rocks  folded  they  were  worn  away, 
though  not  so  rapidly  as  they  were  upfolded.  The  folding 
action  was  so  slow,  that  the  rock  layers  could  be  partially 
reduced  and  the  elevation  of  the  mountains  thereby  greatly 
lessened.  Therefore,  even  before  the  folding  of  a  mountain 
is  finished,  a  large  part  of  its  mass  may  have  been  worn 
away  by  the  agents  of  denudation  (Fig.  229). 

Sculpturirig  of  Mountains.  —  The  carving  of  mountains  is 


PLATEAUS  AND  MOUNTAINS. 


365 


the  result  of  an  extremely  complex  series  of  actions,  and  it 
would  be  impossible  to  adequately  treat  the  subject  in  so 
small  a  book.  There  is  always  a  relation  between  rock 
structure  and  position  ;  and  the  mountain  form  is  the  result 
of  the  interaction  of  the  forces  of  folding  and  of  denudation, 
which  operate  differently  according  to  the  different  positions 
and  kinds  of  rocks.  Some  idea  of  the  topography  that 
results  from  this  interaction  may  be  obtained  from  the 
accompanying  illustrations.     (See  also  Chapter  XXI.) 

The  Drainage  of  Mountains.  —  The  drainage  of  mountains  is 
generally  guided  by  the  rock 
structure,  or  else  by  the  rock 
position.  Valleys  are  liable  to 
be  formed  in  layers  of  rela- 
tively soft  rock,  and  streams 
are  liable  to  have  their  courses 
guided  by  the  ridges  of  the 
mountains.  Therefore  one  of 
the  characteristic  features  of 
mountain  drainage  is  that 
of  parallelism  between  moun- 
tain ridge  and  stream  course  (Fig.  227).  The  tributaries 
to  these  longitudinal  streams,  flow  down  the  valley  sides 
in  direct  courses ;  and  occasionally  the  streams  cross  the 
mountain  ridges  (Fig.  228)  through  deep  and  rather  narrow 
gorges.  It  is  possible  that  in  some  cases  these  transverse 
valleys  are  along  the  courses  occupied  by  the  streams  which 
existed  upon  the  country  before  the  mountains  began  to 
form.  Such  are  known  as  antecedent  valleys,  since  they  had 
their  direction  determined  before  the  mountains  began.  It 
is  believed  that  these  streams  were  able  to  maintain  their 
course  across  the  growing  mountains ;  and  if  this  really  be 
so,  it  is  another  evidence  of  the  extreme  slowness  of  mountain 


Fig.  227. 
A  bit  of  mouutain  drainage. 


366 


PHYSICAL   GEOGRAPHY. 


growth ;  for  if  mountains  are  folded  no  more  rapidly  than 
streams  are  able  to  cut  their  channels,  then  their  growth 
must  be  remarkably  moderate.  Since  there  are  other  possi- 
ble explanations  for  these  transverse  valleys,  we  must  con- 
sider this  explanation  as  merely  an  hypothesis. 

Lakes  are  very 
common  among 
mountains,  their  ori- 
gin in  these  places 
usually  being  the 
folding  of  the  rocks, 
which  form  dams 
across  the  stream 
courses.  By  this  ac- 
tion of  rock  folding, 
streams  may,  in  some 
cases,  be  transformed 
into  lakes  which 
maintain  an  outflow 
in  the  same  direction 
which  the  river  for- 
merly held ;  or,  in 
some  cases,  folding  of 
the  rocks  may  actu- 
ally turn  the  stream 
from  its  course,  and 
make  it  begin  to  cut 
a  valley  at  one  side.  Since  the  origin  of  these  mountain 
lakes  is  that  of  rock  folding,  it  very  often  happens  that 
they  are  exceedingly  deep.  Generally  their  area  is  not 
great ;  but  there  are  some  immense  basins,  the  interior 
basins  previously  described,  which  have  all  the  charac- 
teristics of  lake  basins,  but  which  are  prevented  from  being 


SCALE  OF  MILES 

I 1 I I I I ) 

0    1  2   3  .4    5    6 

Fig.  228. 
Mountain  drainage. 


PLATEAUS  AND  MOUNTAINS. 


367 


occupied  by  lake  water  because  of 
the  slight  rainfall  of  the  region  in 
which  they  exist. 

Destruction  of  Mountains. — It  has 
been  said  that  mountains  are  the  com- 
bined result  of  the  folding  of  rocks 
and  denudation.  When  they  are 
growing,  the  action  of  folding  ex- 
ceeds that  of  denudation,  and  the 
mountains  continue  to  increase  in 
elevation  (Fig.  229).  With  this  in- 
crease, stream  action  and  the  action 
of  weathering  have  their  power  in- 
creased, and  the  mountains  are  very 
rugged.  They  are  rugged  partly 
because  they  are  high,  and  partly 
because  they  are  deeply  carved 
by  stream  erosion.  Therefore  the 
highest  and  most  rugged  mountains 
in  the  world  are  the  youngest;  and 
among  such  mountains,  lakes  are 
usually  present ;  for  the  recent,  or 
perhaps  the  present  folding  of  the 
rocks  has  transformed  a  part  of  the 
streams  into  lakes. 

After  the  folding  has  ceased,  there 
is  no  longer  a  tendency  to  become 
higher  ;  but  the  action  of  denudation 
still  progresses  uninterruptedly,  and 
this  tends  to  constantly  lower  the 
mountains,  and,  in  the  course  of  time, 
to  render  them  less  irregular.  The 
lakes    are    removed,    the    mountain 


c5-  t:- 


CP 


&3 


CO 


c^^il 


-A^tiN-si 


S^ 


\C3iJ 


m 


368 


PHYSICAL   GEOGRAPHY. 


peaks  lose  in  elevation,  the  ridges  are  worn  down,  the  streams 
have  chosen  the  softer  layers  for  their  valleys,  and  the  aspect 
of  the  mountains  has  become  quite  changed.  This  is  the  stage 
which  has  been  reached  by  the  Appalachians.  These  moun- 
tains Avere  once  much  higher  than  now ;  and  since  they  have 
long  been  exposed  to  the  destructive  action  of  weathering 
and  erosion,  they  have  lost  their  ruggedness,  and  are  strik- 


Fig.  230. 
A  mountain  ridge  in  Colorado,  showing  hard  layers  etched  into  relief. 

ingly  in  contrast  with  such  as  the  Rockies,  the  Himalayas, 
and  the  Alps,  which  are  examples  of  young  mountains. 

This  action  of  destruction  may  be  carried  beyond  the  stage 
reached  in  the  Appalachians,  and  whole  mountain  chains 
may  be  worn  down  to  their  very  roots,  and  reduced  to  a 
series  of  relatively  low  hills.  The  highland  portions  of  New 
England,  New  Jersey,  and  the  entire  region  from  this  state 


PLATEAUS  AND  MOUNTAINS.  369 

to  the  Carolinas,  east  of  the  base  of  the  Appalachians,  repre- 
sents such  an  old  mountain  range. 

As  a  result  of  this  mountain  destruction,  many  interesting 
changes  are  brought  about ;  but  the  most  striking  result  is 
the  etching  of  the  surface,  so  that  everywhere_^the  elevations 
are  those  of  hard  rocks,  while  the  depressions  occur  in  the 
soft  strata.  At  first  the  mountain  ridges  ma}^  have  had  for 
their  surface  rock  some  soft  layer  which  was  bent  up  into 
a  ridge  (Fig.  262).  But  after  long  exposure  to  denuda- 
tion, the  soft  layers  are  worn  down  most  rapidly,  and  the 
hard  ones  allowed  to  stand  up  (Fig.  230),  so  that  there  is 
this  final  result  of  relation  between  the  rock  structure  and 
topography.  This  change  may  often  go  so  far  as  to  trans- 
form the  old  mountain  valleys  to  mountain  tops,  and  to 
wear  down  the  original  mountain  ridges  until  they  have 
become  mountain  valleys.  Among  the  Appalachians  there 
are  numerous  instances  of  this  transfer  of  conditions ;  and 
we  then  have  represented  what  are  knoAvn  as  synclinal 
mountains.,  the  nature  of  which  will  perhaps  best  be  under- 
stood by  an  examination  of  Fig.  229,  E. 


REFERENCE   BOOKS. 

Reade.  —  The  Origin  of  Mountain  Ranges.      Taylor  &  Francis,  London, 
1886.     8vo.     21s. 

For  Structure  of  Appalachian  Mountains,  and  an  account  of  experi- 
ments in  mountain  folding,  see  Willis,  Thirteenth  Annual  Report,  U.  8. 
Geological  Survey,  Washington,  1893. 

For  structure  of  Basin  Ranges,  see  Russell,  Fourth  Annual  Report  of 
the  same,  1884. 

2b 


CHAPTER  XX. 

VOLCANOES,  EARTHQUAKES,  AND  GEYSERS. 

Volcanoes  :  Distribution.  —  Nearly  all  of  the  volcanoes  of 
the  earth  are  located  either  in  the  ocean  or  within  a  short  dis- 
tance of  the  coast  (Plate  27).  They  occur  in  lines,  and  are 
very  commonly  present  in  the  highest  mountains,  although 
such  systems  as  the  Himalayas  and  the  Alps  furnish  excep- 
tions to  this.  The  mountains  with  which  they  are  associ- 
ated are  those  in  which  there  is  a  gradual  growth  at  pres- 
ent in  progress.  In  many  cases  they  occur  in  archipelagoes 
near  the  coasts  of  continents.  There  is  a  line  of  recent 
volcanoes,  along  which  there  are  many  still  in  action,  ex- 
tending from  South  America  to  Alaska  :  then  crossing  to  the 
Asiatic  coast,  the  line  continues  down  to  the  East  Indies. 
This  is  the  most  extensive  volcanic  belt  of  the  world. 

The  greater  number  of  the  volcanoes  are  now  found  in  the 
Pacific  or  on  the  borders  of  this  ocean.  Though  there  are 
some  in  the  Atlantic,  this  ocean  is  comparatively  free  from 
them.  Along  the  mid- Atlantic  ridge  there  appears  to  be  a 
line  of  volcanic  action,  and  some  of  the  cones  are  still  in 
eruption.  Iceland  and  Tristan  da  Cunha  are  situated  on 
opposite  ends  of  this  line,  while  the  Azores,  Canaries,  and 
other  islands  are  also  in  the  belt.  Volcanoes  also  occur  in 
other  parts  of  the  earth,  and  there  is  reason  to  think  that 
in  some  places  they  are  present  beneath  the  surface  of  the 
ocean.  Indeed,  volcanic  cones  have  been  known  to  rise 
above  the  sea,  two  instances  of  this  being,  one  in  the  Medi- 
terranean and  the  other  off  the  coast  of  Alaska. 

370 


Face  page  370. 


Approximate  distribution  of  active  and  recent  volcano 


7. 

the  annual  isotherms  of  the  waters  of  the  ocean  surface. 


VOLCANOES,   EABTHQUAKES,  AND   GEYSEBS,        371 

In  the  United  States,  excluding  Alaska,  there  are  now 
no  volcanoes  which  are  known  to  be  in  eruption.  Both  in 
Alaska  and  in  Mexico  there  are  active  cones ;  and  in  the 
northwestern  part  of  the  country,  in  the  state  of  Washington, 
there  are  some  whose  form  is  so  perfect  that  they  may  still 
be  active  volcanoes  in  a  dormant  condition.  Indeed,  there 
are  reports  that  some  volcanoes  in  the  far  west  have  been  in 
eruption  since  the  region  was  inhabited.  While  at  present 
there  is  very  little  if  any  volcanic  activity  in  this  country, 
the  Cordilleras  of  the  west  have  just  passed  from  a  period 
of  most  remarkable  volcanic  action.  There  are  thousands 
of  cones  on  the  plateaus  and  in  the  mountains  of  this  region, 
some  of  them  perfect  in  form,  as  if  still  in  action,  others 
the  nearly  destroyed  remnants  of  cones. 

In  other  parts  of  the  world  there  are  regions  in  which 
there  are  now  no  volcanoes,  but  in  which  there  has  been 
much  volcanic  action  during  the  past  geological  ages.  This 
is  trae  of  the  Auvergne  region  of  central  France,  of  the 
British  Isles,  of  the  east  coast  of  the  United  States,  and 
many  other  places.  On  the  other  hand,  there  are  areas  of 
the  earth  in  which  volcanoes  are  not  only  now  absent,  but 
from  which  they  have  always  been  absent  since  the  begin- 
ning of  the  Cambrian  time.  This  is  true  for  most  of  the 
plains  of  the  Mississippi  valley. 

Materials  Erupted.  —  Steam  is  perhaps  the  most  important 
of  substances  emitted  from  volcanic  vents  (Fig.  231).  This 
is  important  not  merely  because  it  occurs  in  vast  quantities, 
but  also  since  it  is  the  immediate  cause  for  the  volcanic 
eruption.  Of  solid  materials  there  are  two  important  classes, 
the  lava,  which  reaches  the  surface  as  molten  rock  and  then 
cools,  and  the  volcanic  ash  or  pumice,  which  is  really  lava 
blown  full  of  holes  and  made  light  and  porous.  The  pumice 
is  made  into  this  form  by  the  expansion  of  the  steam  which 


372 


PHYSICAL   GEOGRAPHY, 


was  imprisoned  within  it  while  the  molten  rock  existed 
beneath  the  surface  of  the  earth.  Besides  these,  there  are 
less  notable  quantities  of  other  substances,  chiefly  certain 
gases,  such  as  hydrogen,  chlorine,  sulphurous  gas,  etc. 

Some  of  the  steam  passes  into  the  air  as  vapor,  but  much  of 
it  falls  to  the  earth  near  the  volcano,  producing  very  heavy 
rains,  and  often  causing  deluges  in  the  neighborhood  of  the 
cone.     During  an  eruption  there  are  often  violent  thunder 

storms,  in  which 
the  rain  is  largely 
derived  from  this 
source.  When  the 
water  falls  upon  a 
cone  whose  surface 
is  strewn  with  vol- 
canic ash,  the  tor- 
rents of  water 
wash  this  loose 
material  down  the 
hillsides,  and  a 
great  mud  flow  is 
produced.  These 
are  oft^en  very  de- 
structive, and  it  was  such  a  flow  as  this  which  buried  the 
city  of  Pompeii  during  the  eruption  of  79  A.D.  (Fig.  236). 
The  mud  flowed  over  the  houses,  entered  cavities,  and 
formed  casts  of  objects,  thus  protecting  them  from  destruc- 
tion, so  that  in  the  excavations  which  have  been  made  during 
the  present  century,  we  have  obtained  very  perfect  records 
of  the  conditions  under  which  the  Romans  lived  1800  years 
ago. 

The  lava  flow  reaches  the  surface  as  a  mass  of  liquid  rock, 
and  passes   down   the   side    of    the    cone,    often   extending 


Fig.  231. 
Vesuvius  in  eruption,  1872. 


VOLCANOES,   EARTHQUAKES,   AND   GEYSERS.        373 


far  beyond  the  base  and  deluging  the  country  over  which 
it  passes.  It  advances  first  as  molten  rock,  then  a  slight 
crust  forms  over  it,  and  its  motion  becomes  relatively  slow. 
Toward  the  last  of  the  eruption,  the  lava  is  covered  with  such 
a  thick  crust  of  rock  that  one  may  walk  upon  its  surface, 
although  at  the  depth  of  a  few  feet  there  is  still  molten 
lava.  The  surface  of  such  flows  is  extraordinarily  rough; 
for  as  the  liquid  part  moves,  the  solid  crust  is  often  broken 
into    fragments^  (I^ig-     232).      In 

some  rough-surfaced  lava  flows,  it    ,.^  . .     -    — ^.-™~^.^»^, 

is  almost  impossible  for  a  person  to 
travel  over  the  lava  boulders. 

The  lava  does  not  extend  to  a 
very  great  distance  from  the  place 
of  ejection,  for  the  flows  are  rarely 
more  than  20  or  30  miles  in  length. 
Therefore  the  effect  of  a  lava  flow 
is  relatively  local.  In  some  places, 
as  for  instance  in  the  Snake  River 
valley  of  Idaho,  and  in  other  parts 
of  the  plateau  region  of  the  west, 
lava  has  reached  the  surface  through 
great  fissures.  Instead  of  building 
up  a  cone  it   has  welled   out   and 

spread  over  the  surface,  filling  valleys,  and  often  submerg- 
ing hills,  over  areas  of  thousands  of  square  miles.  In  places 
the  lava  fills  the  valleys  to  the  depth  of  2000  or  3000  feet. 

During  an  eruption  in  which  ash  is  sent  to  the'  surface, 
these  light  rock  fragments  are  often  ejected  to  great  heights 
in  the  air,  in  some  cases  apparently  reaching  elevations  of 
several  miles  above  the  surface.  The  heavier  fragments  fall 
back  upon  the  cone,  or  in  its  immediate  neighborhood ;  but 
many  of  the  lighter  fragments  are  sent  so  high  into  the  air. 


Fig.  232. 

Surface  of  a  recent  lava  flow 
in  the  west. 


374 


PHYSICAL   GEOGRAPHY. 


that  before  they  have  been  able  to  fall,  they  are  blown  by  the 
wind  currents  to  a  considerable  distance  from  the  cone. 
In  the  very  violent  eruption  of  Krakatoa,  in  the  Straits  of 
Sunda  (in  1883),  the  finer  particles  of  volcanic  ash  extended 
so  high  into  the  air  that  they  did  not  entirely  reach  the 
earth  for  a  year  or  two.  It  is  estimated  that  the  fragments 
reached  a  height  of  50,000  feet;  and  this  ash  in  the  upper 

layers  of  the  air, 
drifted  over  the  earth 
in  the  prevailing  cur- 
rents, causing  bril- 
liant sunsets  in  both 
Europe  and  America. 
Since  volcanoes  are 
largely  located  either 
in  or  near  the  sea, 
much  of  the  ash  that 
is  erupted,  falls  upon 
the  surface  of  the 
ocean  and  drifts 
about ;  for  pumice  is 
so  light  that  it  will 
float  upon  water. 
After  the  eruption 
of  Krakatoa,  vessels 
sailing  in  the  region  of  the  East  Indies,  often  encountered  so 
much  floating  pumice  that  sailing  was  difficult.  Some  of 
this  is  stranded  upon  the  coast  and  broken  into  small  bits  of 
sand,  but  much  of  it  drops  to  the  bottom  of  the  ocean ;  for 
the  pumice  either  decays  and  breaks  into  fragments,  or  else 
becomes  waterlogged  and  sinks  to  the  bottom. 

Eruptions  of  Volcanoes.  —  There  is  a  great  difference  in 
the  kind  of  eruption  in  different  volcanoes,  and  even  at  dif- 


FiG.  233. 

Lake  formed  by  a  lava  dam,  to  be  seen  in  the 
background. 


VOLCANOES,   EARTHQUAKES,   AND   GEVSEBS.        375 


ferent  times  in  the  same  cone.     On  the  Lipari  Islands,  of  the 

Mediterranean,  there  is  a  small  volcano  which  is  in  almost 

constant   action 

(Fig.  234).  The 

eruptions  are  of 

ash,     and     the 

violence    is  not 

g^eat,    so    that 

sailing     vessels 

may  pass  by  the 

island     without 

danger.     So  far 

as    the    history 

of  these  islands 

is  known,  there 

have    been     no 

real  destructive 

eruptions.       In 

the  case  of  Krakatoa,  on  the  other  hand,  there  has  been  but 

one  eruption  during  the  present  century.     In  the  spring  of 


Fig.  234. 


Fig.  235. 
Diagram  showing  the  disruption  of  Krakatoa. 


376 


PHYSICAL   GEOGBAPHY. 


1883  there  were  signs  of  activity  in  the  volcano,  and  these  in- 
creased until  August,  when  occurred  the  most  remarkable 
eruption  of  recent  times.  One  half  of  the  cone  was  entirely 
blown  away  (Fig.  235) ;  and  where  the  high  volcanic  island 
existed,  there  is  now  deep  water  in  place  of  a  part  of  the 
island.  There  are  numerous  other  instances  of  violent  erup- 
tions, and  in  Iceland  these  are  not  at  all  uncommon. 

Many  volcanoes  have  violent  eruptions  at  one  time,  and 
then  moderate  action.      This  was  the  case  with  Vesuvius, 


Vesuvius,  from  Pompeii. 


Fig.  23(5. 
Monte  Somma  on  the  right,  in  the  background. 


which  was  not  in  eruption  from  the  time  of  the  first  occupa- 
tion of  Italy,  until  the  year  79  a.d.  (Fig.  236).  Then  an 
explosion  took  place  which  was  the  most  vigorous  that  has 
been  experienced  in  the  recorded  history  of  the  cone.  A 
very  considerable  part  of  the  old  mountain,  which  was  known 
as  Monte  Somma,  was  blown  away,  and  a  number  of  towns 
were  destroyed,  including  Pompeii  and  Herculaneum.  Since 
then,  Vesuvius  has  frequently  been  in  eruption,  but  none 
have  equaled  that  of  79. 

Ash-erupting   volcanoes    are   usually   more    violent   than 


VOLCANOES,   EARTHQUAKES,   AND   GEYSEES.        377 

those  which  send  forth  lava.  Of  the  latter  kind,  the  volca- 
noes of  the  Hawaiian  Islands  furnish  excellent  illustration. 
Here  one  may  stand  on  the  margin  of  the  crater  and  look 
upon  a  great  lake  of  molten  rock.  The  surface  of  this 
lake  gradually  rises;  and,  after  several  years,  a  lava  flow 
breaks  through  the  side  of  the  cone  and  flows  down  toward 
the  base,  while  at  the  same  time  the  surface  of  the  lava 
lake  rapidly  descends.  The  eruption  is  not  from  the  crater, 
but  through  fissures  that  are  broken  in  the  side  of  the  cone. 
The  activity  of  these  volcanoes  is  never  excessive. 

The  most  violent  volcanoes  are  those  in  which  there  are 
the  longest  periods  of  rest  between  eruptions.  The  tube 
through  which  the  lava  escapes  becomes  filled  with  solid 
rock,  and  this  appears  to  act  in  a  measure  like  the  closing  of 
the  safety  valve  of  an  engine.  The  steam,  which  is  the 
immediate  cause  for  the  eruption,  finally  accumulates  sufii- 
cient  force  to  blow  out  the  plug,  or  else  to  blow  away  a  part 
of  the  cone. 

Volcanoes  might  be  divided  into  three  groups  upon  the 
basis  of  their  condition.  Some  are  active,  and  their  periods 
of  eruption  are  variable,  in  some  cases  being  many  years, 
in  others  only  a  few  years,  or  even  less  than  a  year  apart 
(Figs.  231,  234-236,  and  239).  A  second  group  is  that 
of  the  dormant  volcanoes,  in  which  there  is  no  present 
sign  of  activity,  but  which  at  any  time  may  break  forth 
in  eruption  (Fig.  238).  Vesuvius  was  a  dormant  vol- 
cano, and  the  inhabitants  of  the  region  believed  it  to 
be  free  from  eruption;  for  towns  and  vineyards  dotted 
the  slopes  of  the  mountain  when  it  began  to  break 
forth  in  the  year  79.  After  this  long  period  of  rest,  the 
length  of  which  cannot  be  estimated,  but  which  certainly 
covered  several  centuries,  Vesuvius  became  an  active  vol- 
cano, and  has  maintained  this  condition  ever  since.     After 


378 


PHYSICAL   GEOGRAPHY. 


aAvhile  any  volcano  will  cease  action  permanently,  and  then 
it  becomes  extinct  (Fig.  237).  The  lesson  taught  by  Vesu- 
vius and  Krakatoa,  should  lead  us  to  include  in  this  group 
only  those  volcanoes  which  have  been  quiet  for  so  long  a 
time  that  there  is  almost  no  possibility  of  eruption.  It  is 
possible  that  some  of  the  supposed  extinct  volcanoes  of  the 
far  west  are  really  dormant  (Fig.  238). 


Fig.  237. 
Mt.  Hood  —  an  apparently  extinct  volcano. 

Form  of  Cone.  —  When  a  volcano  first  begins  to  form,  an 
opening  is  made  in  the  ground,  through  which  ash  and  lava 
are  emitted,  together  with  steam  and  other  gases.  The 
accumulation  of  the  ejected  materials  soon  builds  a  cone 
around  this  orifice.  A  single  eruption  will  suffice  to  form  a 
cone,  the  reason  for  the  conical  shape  being,  that  the  greatest 
quantity  of  material  accumulates  nearest  the  place  of  ejec-.. 


VOLCANOES,    EARTHQUAKES,   AND   GEYSERS.        379 


tion  (Fig.  234).  With  successive  eruptions  the  cone  grows 
higher ;  and  if  they  continue  through  the  same  opening,  tliere 
is  produced  at  the  top,  and  in  the  center  of  the  cone,  a  crater 
which  leads  down  into  the  interior  (Fig.  234). 

If  weathering  and  erosion  were  not  present  to  destroy  the 
conical  form,  in  volcanoes  that  emit  ash  we  would  have  pro- 
duced a  very  per- 
fect cone,  whose 
angle  of  slope 
would  be  as  great 
as  that  assumed 
by  gravel  when  at 
rest.  It  is  proba- 
ble that  this  is 
approximately  the 
form  of  the  cone 
which  is  built  be- 
neath the  surface 
of  the  ocean,  Avhere 
there  is  no  action 
of  denudation. 
On  the  land  there 
is  constantly  a 
tendency  to  re- 
move the  materials 
which  are  building  the  cone.  Instead  of  a  slope  equal  to 
that  of  a  gravel  bank,  the  angle  is  lessened  by  the  washing 
action  of  rain,  and  the  cone  is  gullied  by  stream  valleys. 
In  some  cases,  where  ash  is  ejected  in  great  quantities  and 
frequently,  the  angle  of  slope  is  high,  and  the  form  of  the 
cone  quite  perfect.  In  some  of  the  sharpest  cones  the  angle 
of  slope  is  as  great  as  25°  or  30°.  This  is  illustrated  in 
Popocatapetl    in    Mexico,   and    in    Fusiyama    (Fig.    239). 


f  IG.  238. 
Muir's  Butte,  California  —  a  volcano  recently  in 

eruption. 


380 


PHYSICAL   GEOGEAPHT. 


Violent  eruptions  tend  to  destroy  the  perfection  of  the 
cone;  and  in  the  case  of  Krakatoa,  the  volcano  was  divided 
into  two  parts,  one  of  which  disappeared  into  the  air  (Fig. 


Fig.  239. 
Fusiyama  —  a  Japanese  volcano. 


235).  The  same  is  true  of  Vesuvius,  and  a  part  of  the  old 
rim  which  formed  Monte  Somma  was  blown  away  ;  and 
now  Vesuvius,  as  viewed  from  Pompeii,  shows  a  perfect  cone 


Fig.  240. 

Angle  of  slope  of  volcanoes,  a,  extremely  steep  ash  cone  (approximately  repre- 
sented in  Fusiyama  and  in  submarine  volcanoes);  b,  lava  cone  (Hawaiian 
Islands). 

partly  surrounded  by  a  mountain  wall,  which  is  the  remnant 
of  old  Somma  (Fig.  236). 

The  eruption  of  lava  produces  a  very  much  flatter  cone. 
This   is  well   illustrated   in   the   Hawaiian  Islands,   where, 


VOLCANOES,   EABTHQUAKES,  AND   GEYSEBS.        381 

although  the  volcanoes  are  exceedingly  high,  the  slope  is 
quite  moderate,  being  less  than  10°  (Fig.  240).  This  is  due 
to  the  fact  that  lava  tends  to  flow  away  as  water  does,  and 
consequently  to  broaden  the  cone  as  well  as  to  lessen  the 
slope.  Many  volcanoes  are  at  one  time  erupting  ash  and 
then  lava ;  and  the  cone  produced  is  intermediate  in  form 
between  these  two  extremes.  Such  are  Vesuvius  and  ^tna, 
and  indeed  the  majority  of  the  volcanoes  in  the  world. 

Effects  of  Volcanic  Eruptio7is. — One  of  the  most  important 
effects  of  eruptions  is  the  addition  of  rock  material  to  the 
surface  from  underground  sources.  An  appreciable  part  of 
the  rocks  of  the  crust  have  been  produced  in  this  way. 
Volcanic  action  also  furnishes  heat  to  parts  of  the  earth, 
especially  where  rocks  are  injected ;  and  this  is  one  of  the 
causes  for  hot  springs,  for  many  mineral  veins,  and  for 
the  metamorphism  of  some  rocks.  The  lava  flows  also  in- 
terfere with  the  drainage  of  streams,  sometimes  damming 
them  and  forming  lakes  (Fig.  233),  at  other  times  occupying 
valleys  and  causing  the  streams  to  begin  the  work  of  forma- 
tion of  new  gorges. 

When  eruptions  occur  in  the  ocean,  great  waves  are  pro- 
duced, which  sweep  upon  neighboring  coasts,  and  often 
cause  vast  destruction  of  life.  In  the  East  Indies,  the  low- 
lying  coasts  are  frequently  subjected  to  this  danger  (see  pp. 
178,  179).*  Earthquakes  are  also  produced  as  a  result  of 
volcanic  eruptions  ;  and  both  by  this  indirect  means,  as 
well  as  by  the  lava  and  ash  from  the  eruption,  the  destruc- 
tion of  human  and  animal  life  is  often  very  great.  It  is 
estimated  that  over  50,000  liVes  were  lost  during  the  erup- 
tion of  Krakatoa.  Practically  every  vestige  of  life  was 
extinguished  from  the  island,  and  the  destruction  extended 
to  neighboring  islands. 

Extinct  Volcanoes.  —  When  a  volcano  has  ceased  action, 


382 


PHYSICAL   GEOGRAPHY. 


the  forces  of  denudation  seize  upon  the  cone  and  wear  it 
away.  At  first  the  regularity  of  the  cone  is  destroyed  by 
the  gullying  action  of  streams  (Figs.  237  and  241),  then 
its  size  decreases,  and  finally  merely  a  remnant  of  it  is  left. 
This  remnant  is  always  that  of  the  central  part  of  the  cone, 
partly  because  this  is  the  divide  and  hence  less  exposed  to 


Fig.  241.  ^ 

Mt.  Shasta  on  the  left ;  Shastina,  a  more  recent  cone,  on  the  right. 


erosion,  but  mainly  because  it  is  the  place  where  the  hardest 
rock  occurs.  The  old  vent  or  tube  of  the  volcano  is  filled 
with  rock  from  the  last  eruption  that  has  occurred ;  and  since 
this  is  less  porous  than  the  lava  or  ash  that  forms  the  cone 
itself,  it  is  much  more  resistant  to  weathering.  These  necks 
or  plugs  (Fig.  242)  of  volcanoes  are  present  in  all  regions 


VOLCANOES,  EARTHQUAKES,  AND   GEYSERS.        383 


t--i 


an  old  volcanic 


where  volcanic  action  has  recently  ceased.    Upon  the  western 
plateau  there  are  thousands  in  all  stages  of  destruction. 

As  the  volcano  disappears,  denudation  reaches  places  into 
which  lava  has  been  intruded  in  the  form  of  dykes  or  bosses; 
and  when  these  are  harder  than  the  surrounding  rock,  they 
stand  up  as  ridges.     With 
the  wearing  away  of  the     f 
surface,  the  lava  flows  also 
disappear ;  and  where  they 
are  harder  than  the  rocks 
upon  which  they  rest,  they 
often  protect   these    from 
destruction,    causing   flat- 
topped     hills     and     small 
table-lands.     These    lava- 
capped     huttes     or     mesas 
(Fig.   218)  are  very  com- 
mon  in    the    regions    be- 
tween the  Rocky  Mountains  and  the  Pacific  coast. 

Cause  of  Volcanoes.  —  The  immediate  cause  of  volcanic 
eruptions  is  the  presence  of  steam ;  and  in  a  measure  the 
eruption  may  be  compared  to  the  bursting  of  a  boiler. 
There  is  steam  present  in  a  superheated  condition,  this  tends 
to  find  relief,  and  the  eruption  occurs.  The  origin  of  tlie 
heat  which  causes  the  melting  of  the  rock  cannot  be  stated. 
It  has  to  do  with  the  heated  condition  of  the  earth,  and 
since  we  are  not  certain  just  what  this  condition  is,  we  of 
course  are  not  able  to  state  what  causes  the  molten  rock. 
The  same  cause  that  produces  the  folding  of  mountains 
appears  to  operate  in  the  formation  of  volcanoes;  and  the 
volcanic  action  is  in  most  cases,  if  not  in  all,  an  indication 
that  the  crust  is  folding. 

Earthquakes.  —  By  far  the  greater  number  of  earthquakes 


Fig.  242. 

Mato  Tepee,  Wyoming - 
neck. 


384 


PHYSICAL   GEOGRAPHY, 


Fig.  2i3. 

The  earthquake  wave.    E,  epicen- 
trum.    F,  focus. 


X 


occur  either  near  volcanoes  or  among   mountains,   though 

some  have  occurred  at  great  distances  from  either  of  these. 

The  earthquake  is  a  jarring 
of  the  rocks,  caused  by  some 
shock  which  is  transmitted  as  a 
series  of  spherical  waves  in  all 
directions  through  the  strata. 
The  point  of  origin  of  the 
shock  is  known  as  the  focus 
(Fig.  243),  and  from  this  cen- 
ter the  earth  waves  move  in  all 

directions.     If  the  rocks  were  of  uniform  texture,  the  earth- 
quake waves  would  have 

a    spherical    form;     but      'V^  ';.'W^ 

since  the  strata  vary  in 

character,  the  rate  of  mo- 
tion differs,  and  conse- 
quently    the      spherical 

form  is    distorted    (Fig. 

244). 

The  point  on  the  earth's 

surface  directly  above  the 

focus   is   known   as    the 

epicentrum^    and    this    is 

the     place     where     the 

shock    first    reaches    the 

surface.    The  waves  come 

from  the  earth  at  equal 

distances  from  this  point, 

and   on   all  sides   of    it. 

If  the  rock  texture  were 

uniform,  the  shock  would 

be  felt  at  the  same  time  at  all  points  whose  distance  from  the 


■•*X:' 


'^\ 


>-;/ 


%, 


Fig.  244. 
Earthquake  waves  of  Charleston  earthquake, 
showing  effect  of  folded  rocks  of   Appala- 
chians. 


VOLCANOES,   EARTHQUAKES,   AND  GEYSERS.        385 


epicentrum  is  the  same.  The  most  violent  part  of  the 
earthquake  is  in  the  immediate  vicinity  of  the  center,  while 
it  decreases  quite  uniformly  away  from  this  (Fig.  245). 

Even  during  violent  earthquakes,  the  amount  of  movement 
of  the  rocks  is  not  very  great;  but  the  effects  of  the  jar  are 
often  very  disastrous.  Parts  of  cliffs  are  thrown  down, 
landslides  produced, 
houses  destroyed  (Fig. 
246),  trees  overturned, 
and  general  destruction 
caused.  The  destruc- 
tion of  human  life  is 
greatly  increased  by  the 
fact  that  houses  are 
readily  throAvn  down 
by  earthquake  waves. 
When  earthquake 
shocks  occur  in  the 
ocean,  great  sea  waves 
are  often  produced,  and 
these,  sweeping  upon 
the  coasts,  devastate  the 
loAvlands. 

Any  jar  in  the  earth 
will  produce  an  earth-  yig.  2W. 

quake.       During  the  ex-  Earthquake  shock  in  Japan. 

plosion  of  dynamite  at  Hell  Gate,  near  New  York,  a  few 
years  ago,  a  shock  was  started  which  was  measured  as 
far  away  as  Washington  on  the  one  side,  and  Boston  on  the 
other.  The  great  earthquake  shocks  are  evidently  connected 
either  with  volcanic  eruptions  or  with  faulting  in  the  rocks. 
The  violent  eruption  of  a  volcano,  like  that  of  Krakatoa, 
sends  a  series  of  earthquake  waves  through  the  rocks ;  and  in 
2c 


386 


PHYSICAL   GEOGRAPHY. 


the  time  immediately  preceding  volcanic  eruptions,  earth- 
quake shocks  are  very  common,  being  apparently  the  result 
of  unsuccessful  efforts  of  the  lava  to  force  its  way  to  the 
surface.  As  the  rocks  are  broken  apart,  each  step  in  its  prog- 
ress toward  the  surface  produces  a  jar.     When  mountains 


Fig.  246. 
Effect  of  earthquake  in  Japan,  1891. 

are  being  formed,  rocks  are  often  broken  and  faulted;  and  as 
they  break  and  slip,  waves  are  started  which  produce  earth- 
quake shocks  (Fig.  247).  Many  of  the  most  violent  earth- 
quakes of  the  world  appear  to  be  attributable  to  this  cause. 

Geysers  and  Hot  Springs — Underground  water,  after  a 
passage  through  the  earth,  often  finds  its  way  back  to  the 
surface  in  a  heated  condition.  In  such  cases  hot  springs 
are  produced,  and  these  are  generally  mineral  springs ;  for 


VOLCANOES,    EABTHQUAKES,   AND   GEYSERS.        387 

hot  water,  in  passing  through  the  crust,  finds  many  mineral 
substances  which  it  can  dissolve  (Fig.  105).  Hot  springs 
are  quite  commonly  found  in  association  with  volcanoes; 
and  it  is  very  probable  tliat  the  heat  of  the  water  is  in  most 
cases  furnished  by  some  supply  connected  with  volcanic  ac- 


FiG.  247. 

Breaking  of  the  earth  along  the  fault  line  which  caused  the  Japanese 

earthquake  shock  of  1891. 

tion.     Even  after  volcanoes  have  ceased  activity,  hot  springs 
may  remain  in  the  neighborhood. 

Sometimes  hot  springs  have  the  peculiar  habit  of  bursting 
forth  into  eruptions  of  steam  and  hot  water,  and  then  a  gey%er 
is  produced  (Fig.  249).  A  geyser  may  be  defined  as  a  hot 
spring  which  has  a  habit  of  intermittent  eruption.  One  of 
the  geysers  of  the  Yellowstone  Park  region,  the  Artemesia, 
was  for  a  long  time  known  as  a  hot  spring,  and  then  suddenly 


888 


PHYSICAL   GEOGBAPHY. 


began  eruptions  like  tlie  other  geysers  of  the  Park.  While 
hot  springs  are  very  widely  distributed,  geysers  are  quite 
uncommon.  There  are  only  three  places  in  the  world  where 
they  are  features  of  importance,  one  being  the  Yellowstone 
Park,  the  second  in  Iceland,  and  the  third  in  New  Zealand. 
In  all  of  these  cases,  the  geysers  are  bringing  to  the  surface 


Fig.  248. 
Crater  of  Oblong  Geyser,  Yellowstone  Park. 


large  quantities  of  chemically  dissolved  mineral  matter ; 
and  in  the  Yellowstone  region,  craters  are  built  around  the 
geyser  (Fig.  248). 

There  is  much  difference  in  the  time  between  the  eruptions 
of  geysers,  some  being  in  eruption  every  few  hours,  others  hav- 
ing very  irregular  periods  of  action.  Hot  water  sloAvly  boils 
in  the  tube,  then  it  overflows  gently,  and  suddenly,  Avith  very 


VOLCANOES,   EARTHQUAKES,   AND   GEYSERS.        389 


little  warning,  bursts  forth  into  eruption,  when  the  air  is 
filled  with  a  great  column  of  hot  water  and  steam,  which  in 
the  case  of  the  larger  gey- 
ser usually  rises  to  a 
height  of  100  or  200  feet 
(Fig.  249). 


-*<>•- 


REFERENCE   BOOKS. 


VOLCANOES. 


Dana.  —  Characteristics  of 
Volcanoes.  Dodd,  Mead  & 
Co.,  New  York,  1891.  8vo. 
$5.00.  (A  very  complete  and 
valuable  discussion  of  the  sub- 
ject.) 

Hull. — Volcanoes:  Past  and 
Present.  Scribner,  New  York 
(Contemporary  Science  Se- 
ries), 1892.     12mo.     $1.25. 

Judd. — Volcanoes.  Appleton 
&  Co.,  New  York  (Inter- 
national Scientific  Series), 
1881.     12mo.     $2.00. 

For  Eruption  of  Krakatoa,  see  The  Eruption  of  Krakatoa  (edited  by 
Symons).     Trubner  &  Co.,  London,  1888.    4to.     30s. 

For  Hawaiian  Volcanoes,  see  Dutton,  Fourth  Annual  Report,  U.  S. 
Geological  Survey,  Washington,  1884. 


Fig.  249. 
Old  Faithful  Geyser,  Yellowstone  Park. 


earthquakes. 

Milne.  —  Earthquakes.     Appleton,  New  York,  1891   (International  Scien- 
tific Series),     12rao.     $1,75. 

For  a  description  of  the  Charleston  Earthquake  of  1886,  see  Dutton, 
Ninth  Annual  Report,  U.  S.  Geological  Survey,  Washington,  1889.1 


1  Nearly  all  of  these  articles  in  the  IT.  S.  Geological  Survey  Keports  are  well  illustrated  ;  and 
since  many  of  them  are  readily  obtained  free  of  cost,  they  should  be  widely  used. 


CHAPTER    XXI. 

THE   TOPOGRAPHY   OP   THE  LAND. 

General  Statement. — Land  forms  are  of  two  kinds:  (1)  those 
that  have  been  huilt  by  some  agency  and  (2)  those  that 
have  resulted  from  the  combined  action  of  building  and 
carving.  By  far  the  greater  number  of  land  forms  are  of 
the  last  origin,  and  there  are  fcAv  that  have  resulted  exclu- 
sively from  constructive  action.  There  are  two  sets  of 
forces  working  upon  the  earth  in  an  effort  to  modify  its 
surface  :  the  one  internal,  which  tends  to  make  the  surface 
diverse,  the  other  mainly  external  and  tending  to  level.  As 
a  result  of  the  action  of  the  former,  the  earth's  surface  is 
thrown  into  a  series  of  waves,  great  and  small,  and  some  of 
these  are  even  now  in  process  of  formation. 

If  nothing  had  interfered,  these  earth  waves  would  have 
made  the  surface  very  irregular,  and  the  mountain  chains 
would  have  risen  to  vastly  greater  heights,  and  often  with 
much  steeper  slopes  than  we  really  find.  In  opposition  to 
this  force  there  are  the  agents  of  denudation,  which  derive 
their  power  chiefly  from  causes  outside  of  the  earth  itself, 
and  are  mainly  manifestations  of  solar  energy,  combined  with 
complex  causes,  some  of  which  are  described  in  the  first 
chapters  of  the  book.  By  removing  materials  from  the  higher 
parts  and  spreading  them  over  the  lower  areas,  the  agents 
of  denudation  are  engaged  in  the  work  of  leveling.  In 
the  course  of  this,  it  is  often  necessary,  or  most  easy,  to 
temporarily  increase  the  irregularities,  as  is  done  by  the  Col- 
orado in  its  work  of  valley  formation  in  the  great  Arizona- 

390 


THE  TOPOGBAPHY  OF  THE  LAND. 


391 


Utah  plateau  (Plate  28).  The  present  land  form  is  the 
result  of  the  complex  interaction  of  these  forces,  and  it  is 
still  in  process  of  change. 


Plate  28. 
Brink  of  Marble  Canon,  Colorado  River. 


392  PHYSICAL   GEOGBAPHY. 

Some  parts  of  the  earth  are  now  being  built  up,  others  are 
being  worn  down  by  one  cause  or  another.  As  a  result  of 
this,  the  surface  of  the  earth  presents  most  complex  features  ; 
but  if  we  look  at  the  causes  and  influences  that  are  at  work, 
it  becomes  a  much  more  simple  task  to  account  for  them. 
These  may  be  briefly  summed  as  follows  :  The  crust  of  the 
earth  is  in  movejtient,  in  some  places  upward,  in  others  down- 
ward, here  by  broad  uplift  or  downsinking,  there  by  the  more 
local  and  intense  upfolding  or  downfolding  which  accompa- 
nies mountain  growth.  Some  regions  are  therefore  nat- 
urally high,  others  low ;  some  are  mountains,  others  plains, 
and  still  others  plateaus.  Denudatioyi  is  everywhere  at 
work ;  and  since  the  conditions  are  variable,  the  results  are 
quite  different.  Its  action  upon  plains  differs  from  that 
upon  plateaus ;  and  in  regions  of  horizontal  strata,  its  effect 
is  quite  different  from  that  produced  when  the  rock  position 
or  attitude  is  complex.  Not  merely  does  the  difference  in 
rock  position  produce  a  perceptible  effect,  but  the  variations 
in  resistance  to  weathering  and  erosion  are  of  most  funda- 
mental importance.  These  agents  of  denudation  are  also 
engaged  in  the  work  of  construction ;  for  the  materials 
taken  from  one  place  find  rest  in  another,  and  often  the  two 
processes  of  tearing  down  and  building  up  overlap. 

Constructive  Land  Forms  :  By  Internal  Forces.  —  It  is  to 
be  borne  in  mind  that  in  nearly  every  part  of  the  land,  no 
matter  what  the  origin  of  the  surface  features,  there  is  evi- 
dence of  the  action  of  the  destructive  denudation ;  and  there- 
fore in  this  section  we  deal  merely  with  the  skeleton,  not 
with  the  perfected  form.  The  larger  diversities  of  the 
earth's  surface,  although  greatly  sculptured,  owe  their  main 
features  to  the  action  of  contraction  of  the  earth's  interior.  ^ 

1  Accepting  the  contractional  hypothesis,  as  we  may  fairly  do,  for  a  work- 
ing hypothesis. 


THE  TOPOGRAPHY  OF  THE  LAND.  393 

Thus  the  continents  and  mountains,  considered  without  ref- 
erence to  details,  are  true  constructional  forms,  being  built 
by  the  folding  of  the  rocks.  In  the  same  way,  many  of  the 
plateaus,  such  as  those  which  lie  at  the  base  of  the  Rocky 
Mountains,  are  due  to  the  elevation  of  a  part  of  the  earth's 
crust ;  and  many  plains  have  also  been  given  their  present 
condition  by  land  movements.  This  is  the  case  with  the 
coastal  plain  which  forms  the  eastern  margin  of  the  country 
south  of  New  York.  This  represents  an  old,  nearly  level 
sea  bottom,  very  recently  raised  into  the  condition  of  land ; 
and  another  elevation  of  this  part  of  the  continent  to  a 
height  of  600  feet,  would  add  a  plain  which  in  some  places 
would  be  more  than  100  miles  in  width. 

The  volcano  is  also  a  constructional  form  dependent  upon 
the  heated  condition  of  the  rocks  beneath  the  crust  (Figs. 
234-241).  It  is  built  up  and  is  formed  into  a  typical  topo- 
graphic feature ;  but  under  different  circumstances  this  form 
varies  somewhat.  The  cone  results  from  the  piling  up  of 
materials  derived  from  beneath  the  crust,  and  accumulated 
into  a  conical  heap  around  the  place  of  ejection.  Partly 
because  of  denudation,  and  partly  because  of  the  explosive 
action  of  some  eruptions,  the  cones  are  much  less  perfect  than 
they  normally  tend  to  be. 

By  Agents  of  Denudation.  —  Some  topographic  features  are 
produced  directly  by  the  building  action  of  the  agents  of 
denudation ;  but  these  are  usually  of  minor  importance. 
As  a  cliff  crumbles  away,  talus  deposits  accumulate  at  its 
base,  and  these  often  produce  great  sweeping  slopes  at  the 
foot  of  steeply  rising  mountains  (Figs.  118  and  219).  Some- 
times this  curve  unites  with  that  caused  by  denudation,  and 
a  double  curve  is  then  produced.  The  wind  often  blows 
sand  into  mounds,  and  these  may  cover  great  areas,  com- 
pletely burying  the  underlying  topography.     These  are  par- 


394  PHYSICAL   GEOGRAPHY. 

ticularly  liable  to  be  formed  near  seacoasts  (Fig.  120);  but 
sand-dune  areas  are  also  common  in  arid  regions. 

When  filled  with  sediment,  and  transformed  to  swamps  or 
plains  (Fig.  172),  constructional  forms  of  monotonous  regu- 
larity are  often  built  in  the  site  of  lakes.  The  same  condition 
results  when  lakes  are  displaced  by  other  causes,  as  is  the  case 
when  they  evaporate  ;  and  many  of  the  great  alkaline  plains 
or  flats  of  the  Great  Basin  are  old  lake  bottoms  (Fig.  150). 
The  disappearance  of  a  glacial  lake  often  leaves  an  exten- 
sive plain,  as  is  so  well  illustrated  in  the  great  wheat  plains 
of  the  valley  of  the  Red  River  of  the  North  (Fig.  215). 
In  these  cases  the  shore  lines  are  also  left,  and  these  topo- 
graphic forms,  though  of  minor  importance,  are  often  strik- 
ing features  in  the  landscape  (Fig.  170).  Deltas  (Fig. 
154),  bars  (Fig.  213),  and  spits  (Fig.  196)  are  built  up  in 
the  lake  waters ;  and  upon  the  disappearance  of  the  lake 
these  are  left  upon  the  valley  sides  (Fig.  170). 

Rivers  also  build  deposits,  the  most  notable  being  deltas 
and  floodplains  ;  but  in  somes  cases,  terraced  valley  sides 
result  from  the  constructive  action  of  the  river  floods.  One 
of  the  most  important  causes  for  the  details  of  the  topog- 
raphy in  northern  United  States,  is  found  in  the  recent 
glaciation  ;  and  much  of  this  topographic  variety  is  due  to 
the  building  action  of  the  ice.  With  the  debris  that  it  car- 
ried, the  glacier  formed  great  plains,  either  by  direct  depo- 
sition from  the  ice,  or  in  a  secondary  way  through  the 
intervention  of  water  produced  by  ice  melting.  ]\Iuch  of  the 
prairie  country  of  the  Central  States  owes  its  present  levelness 
to  these  causes.  In  other  places,  hills  of  peculiar  and  irregu- 
lar form  were  built  by  the  ice.  To  the  majority  of  people  who 
live  in  the  glaciated  belt,  these  hills  of  gravel  and  unstrati- 
fied  till  must  be  familiar  features;  and  in  the  morainal  regions 
they  are  strikingly  developed.     (See  Chapter  XVII.) 


THE   TOPOGRAPHY  OF  THE  LAND.  395 

The  ocean  is  the  great  receiving  ground  for  the  waste  of 
the  land ;  and  for  the  most  part  the  debris  is  spread  quite 
evenly  over  the  bottom,  producing  a  plain,  which  in  some 
cases  is  partly  raised  above  the  sea.  But  along  the  shore 
line,  the  constructive  action  of  the  ocean  is  producing  many 
irregularities,  though  here,  as  elsewhere,  the  actions  of  tear- 
ing down  and  building  up  are  so  intimately  associated  that 
it  is  often  difficult  to  draw  the  line  between  them.  Still, 
the  beaches  (Figs.  200  and  201),  the  bars,  the  long  sandy 
islands  (Fig.  194),  and  other  similar  coastal  features,  are 
often  mainly  the  result  of  the  action  of  the  waves  and  cur- 
rents in  building  up  materials  furnished  by  various  means. 
When,  for  any  reason,  the  level  of  the  sea  is  changed  in  its 
relation  to  the  land,  these  shore-line  formations  are  either 
submerged,  or,  if  the  land  rises,  are  left  as  ancient  shore 
lines,  which  then  resemble  those  remaining  when  lakes  dis- 
appear. 

Bf/  Animal  and  Playit  Life. — In  various  ways,  both  animals 
and  plants  are  engaged  in  constructing  land  forms.  The 
salt  marsh  of  the  seashore  (Fig.  206),  and  the  swamps  of 
the  land,  in  part  represent  this  action  ;  but  the  most  notable 
action  of  life  in  this  respect  is  that  of  the  corals,  which  are 
building  reefs  (Fig.  207).  It  is  true  that  the  corals  do  not 
build  the  reefs  above  the  sea  level ;  but  a  slight  elevation  of 
the  bottom  has  often  raised  them  to  the  air.  Also,  the 
action  of  the  waves  may  pile  the  coral  fragments  above 
the  reach  of  ordinary  waters  ;  and  wind  action,  by  blowing 
the  coral  fragments  into  dune-like  hills,  then  causes  them  to 
rise  still  higher.  By  these  constructive  processes  combined, 
many  islands  are  built  in  the  sea  (Figs.  208  and  209). 

Effect  of  Rock  Structure  upon  Topography. — The  land  forms 
constructed  in  the  ways  above  described,  are  subjected  to 
attacks  from  all  the  agents  of  denudation  ;  and  as  a  result 


396  PHYSICAL   GEOGRAPHY. 

of  this,  the  land  surface  presents  many  diversities.  Under 
uniform  conditions,  denudation  affects  rocks  differently 
according  to  (1)  their  elevation,  (2)  their  position,  and 
(3)  their  structural  features.  Moreover,  the  intensity  of 
denudation  varies ;  and  as  a  result  of  these  facts,  land  forms 
differ  from  place  to  place.  It  is  impossible  here  to  enter  into 
this  subject  in  any  considerable  detail ;  but  some  of  the 
main  principles  may  be  briefly  stated. 

Much  depends  upon  the  ease  with  which  materials  may  be 


Fig.  250. 

View  in  Brazil,  showing  hard  layer  etched  into  relief  by  the  removal  of  the  less 

resistant  enclosing  rocks. 

removed  (Fig.  250).  In  high  mountains,  where  the  grade 
is  steep  and  denudation  intense,  the  etching  of  the  rocks  is 
very  sharply  done  (Figs.  224,  225,  230,  and  261) ;  and  hence, 
in  such  places,  we  have  the  characteristic  ruggedness  of  high 
mountains  ;  but  when  the  mountains  are  low,  even  though 
the  differenc3  in  rock  hardness  may  be  great,  the  outlines 
are  less  angular  and  more  rounded  and  floAving.  In  this 
connection  one  may  contrast  the  Alps  (Figs.  143,  144,  and 
220)  with  the   Highlands   of   Scotland,  or  the  Rockies  of 


THE  TOPOGRAPHY  OF  THE  LAND, 


397 


Plate  29. 


Navajo  Church,  Arizona,  showing  sharpness  of  denudation  in  an  arid  region. 
Soft  clay,  capped  by  harder  rock,  in  foreground. 


398 


PHYSICAL   GEOGRAPHY. 


Colorado  (Figs.  221  and  223)  with  the  Appalachians  or  the 
Adirondacks  (Figs.  263  and  264). 

With  conditions  of  aridity,  the  soil  covering  is  readily 
removed  from  the  rocks,  so  that  they  are  exposed  to  the  air  ; 
and  hence,  here  also,  angularity  and  ruggedness  of  topography 
prevail  (Figs.  122  and  142  and  Plates  28  and  29).  Often- 
times the   streams  cannot  carry  the  material   furnished  to 

them,  and  instead 
of  trenching  the 
highlands,  they 
flow  on  the  sur- 
face of  a  plateau. 
This  is  the  case 
with  the  river 
Platte.  The  in- 
tensity of  denuda- 
tion is  therefore 
of  great  impor- 
tance, and  this 
varies  with  the 
stage  of  develop- 
ment^ so  that  there 
is  an  intimate 
relation  between 
topography  and 
the  age  of  topo- 
graphic forms. 
The  young  valley  is  a  sharply  defined  feature  (Fig.  133), 
while  the  mature  valley,  in  which  the  intensity  of  erosion 
has  ceased,  is  rounded  under  the  more  widespread,  but  more 
moderate  action  of  weathering  (Fig.  135).  Altitude  is  an 
important  element  in  this  connection,  but  it  is  by  no  means 
the  only  one. 


Fig.  251. 
A  cliff  in  the  Yosemite. 


THE   TOPOGRAPHY  OF  THE  LAND. 


399 


Much  depends  upon  the  rock  structure.  Even  though  it 
be  soft,  a  rock  of  uniform  texture  produces  massive  effects. 
The  granite  of  the  Yosemite  (Figs.  164  and  251)  is  com- 
posed of  materials  uniformly  arranged,  —  hence  the  bold, 
regular  outlines.  Massive  beds  of  limestone  produce  the 
same  effect ;  and  among  some  of  the  ranges  of  the  Rocky 
Mountains,  where  the  rock  is  a  thick  bed  of  limestone  of 
quite  uniform  texture,  there  are  places  of  great  precipitous- 


^■jj^^v,''^^,. 


..'T^IS 


-^^/•^x^ 


'ffV-A^»^^'-      **- 


Fig.  252. 
Cliffs  in  the  loess  clay  of  China. 


ness.  Even  when  the  surface  is  covered  with  consolidated 
clay,  this  uniformity  impresses  itself  upon  the  topography, 
as  is  so  well  illustrated  in  the  Chinese  region  (Fig.  252). 
Upon  the  seashore  these  massive  rocks  are  often  cut  into 
cliffs,  which  frequently  rise  to  great  heights,  as  in  the  case  of 
the  chalk  cliffs  of  England. 

On  the  other  hand,  if  the  rock  is  in  layers,  or  if  for  any 
other  reason  it  is  rendered  mechanically  weak  in  places,  the 
boldness  disappears.     On  the  seacoast,  the  weakness  of  the 


400 


PHYSICAL   GEOGRAPHY. 


rocks  is  taken  advantage  of  by  the  waves,  and  the  weak 
places  indicated  by  an  indentation  in  the  coast.  Where  the 
rocks  are  jointed  or  broken,  or  where  one  layer  is  softer  than 
another,  sea  caves  (Fig.  198),  chasms  (Figs.  199  and  253), 

and  even  small  bays,  may  be 
produced.  The  cliffs  are  not 
so  high  nor  so  angular  as  in 
the  massive  rocks  (Figs.  254 
and  255);  for,  both  by  the  waves 
i  and  by  weathering,  they  are 
caused  to  crumble  and  to  as- 
sume a  more  gentle  slope. 

Since  hard  strata  (meaning 
those  resistant  to  denudation) 
are  worn  down  with  much  less 
rapidity  than  soft  ones,  where 
these  alternations  exist  in  such 
a  position  as  to  be  exposed  to 
denudation,  there  is  much  ir- 
regularity introduced.  Accord- 
ing to  the  attitude  of  the  rocks, 
there  is  much  variety  in  the 
topography.  If  the  strata  are 
horizontal,  the  hard  layers  tend  to  remain  ;  and  betAveen 
the  rivers,  there  are  relatively  flat-topped  hills,  capped  by 
these  hard  rocks.  Their  margins  are  steeply  sloping,  but 
the  slope  decreases  where  the  layers  are  soft  (Figs.  256 
and  257).  These  features  of  the  land  are  particularly  well 
developed  in  arid  regions,  where  differences  in  rock  hardness 
are  always  etched  with  greater  intensity  than  in  moist  coun- 
tries; and  in  such  places  terraces  are  often  produced.  These, 
which  have  been  called  terraces  of  differential  degradation, 
are  flat-topped  where  hard  layers  exist,  while  betw^een  two 


Fig.  253. 

Rafe's  Chasm,  Cape  Ann,  Mass 
wave-worn  chasm  in  granite 


THE  TOPOGRAPHY  OF  THE  LAND, 


401 


J^'IG.  254. 
A  rugged  coast  in  massive  granite,  Cape  Ann,  Mass. 

such  areas  there  is  a  steep  ascent.  In  such  a  place,  in 
traveling  across  country,  one  passes  over  a  series  of  steps 
on  the  land   (Fig.    217  and  Plate  28).      Such  topography 


Fig.  255. 
A  granite  coast  where  the  rock  is  much  jointed,  Cape  Ann,  Mass. 
2d 


402 


PHYSICAL   GEOGRAPHY. 


is  typical  of  plateaus,  and  particularly  of  those  in  arid  lands. 
On  the  seashore,  the  tendency  to  produce  a  step-like  coast 
exists  where  the  horizontal  rocks  outcrop  in  cliffs  composed 

of  layers  of  different  hard- 
ness. 


With     gently     dipping 
rocks,  very  nearly  the  same 
kind     of      topography     is 
produced ;    but    the    flat- 
topped  areas  are  less  dis- 
tinct.    In  passing  across  a 
country  in  which  the  differ- 
ences in  hardness  of  slightly  inclined  rocks  are  well  brought 
out,  as  in  the  central  part  of  Texas,  the  aspect  of  the  country 
changes  entirely,  according  to  the  direction  pursued.     If  one 


Fig.  256. 

Effect  of  hard  layers  (unshaded)  in  the 
denudation  of  nearly  horizontal  strata. 


pi'm&ii:::imim 


Fig.  257. 
Signal  Butte,  Texas.    An  outlying  hill  protected  by  a  hard  cap  of  horizontal  rock. 

travels  at  right  angles  to  the  dip,  he  may  pass  for  long  dis- 
tances upon  a  flat-topped  terrace,  bounded  on  one  side  by  a 
steeply  rising  face,  and  on  the  other  by  a  steeply  descending 


THE  TOPOGRAPHY  OF  THE  LAND. 


403 


slope  (Fig.  258).     If  going  in  the  direction  of  the  dip,  one 
iiscends  a  steeply  sloping  hill,  then  passes  over  a  bench  to 

H 


Fig.  259. 
H,  hard  stratum. 


Fig.  258. 

Step  topography  in  region  of  inclined  strata.    H,  H,  H,  hard  layers ;   S,  S,  S, 

soft. 

another  sloping  hill,  and  this  may  be  repeated  many  times. 
If  the  journey 
is  in  the  oppo- 
site direction, 
there  are  a  se- 
ries of  descents 
with  interme- 
diate terraces. 
Looking  in  the 
direction  of  the 

dip,  one  sees  a  series  of  hills,  while  the  view  in  the  opposite 

direction  is  over 
the  surface  of 
the  plain.  The 
flat  areas  are 
determined  by 
hard  layers,  and 
the  steep  slopes 
are  also  due  to 
their  presence ; 
for  they  serve 
to  protect  the  softer  underlying  layers  from  destruction. 
Where  such  a  series  of  rocks  occurs  on  the  seacoast,  the 


Fig.  260. 
H,  hard  stratum ;  S,  soft. 


404 


PHYSICAL   GEOGRAPHY. 


form  of  the  coast  differs  entirely  according  to  the  direction 
of  the  dip.  If  the  waves  beat  against  a  series  of  rocks  dip- 
ping toward  the  sea,  they  produce  a  gently-sloping  shore, 
whose  form  and  position  are  determined  by  a  hard  layer 
(Fig.  259).  On  the  other  hand,  if  the  dip  is  aAvay  from  the 
sea,  the  waves  beat  against  a  bluff  (Fig.  260.) 

When  the  strata  are  inclined  at  a  high  angle,  the  hard 


Fig.  261. 
A  ridge  of  hard  rock  etched  into  relief  by  more  rapid  removal  of  softer  strata. 

layers  tend  to  stand  up  above  the  surrounding  country  in  the 
form  of  ridges,  while  the  position  of  the  softer  strata  is  indi- 
cated by  valleys  (Figs.  230  and  261).  These  peculiarities 
are  particularly  well  illustrated  among  mountains,  where  the 
ridges  and  peaks  are  quite  commonly  the  result  of  the  resist- 
ance of  some  hard  layer  which  is  tilted  into  the  mountain 
form  (Figs.  219,  225,  and  230).    Many  complexities  of  moun- 


THE  TOPOGRAPHY  OF  THE  LAND. 


405 


tain  topography  are  the  result  of  this  etching  of  folded 
rocks  which  present  differences  in  hardness.  This  is  seen 
among  the  Appalachians  (Fig.  262),  where  nearly  all  of  the 
ridges  are  made  of  hard  strata,  and  where  they  form  ridges 
because  they  are  more  resistant  than  the  surrounding  rocks. 
Not  merely  are  there  ridges  where  hard  layers  exist,  but 
peaks  (Fig.  220)  are  often  produced  where  unusually  hard 
rocks  are  found  ;  and  very  often,  where  the  general  rock 
structure  is  harder  than  that  of  the  surrounding  regions, 
these  places  stand  up  as  more  elevated  areas.  Thus  the 
Adirondacks,  the  New  England  area,  etc.,  are  high  mainly 
because  their  rocks  are 
prevailingly  hard.  When, 
by  land  movements,  these 
carved  areas  are  brought 
beneath  the  sea,  their 
irregularities  impress 
themselves  upon  the 
coast  line,  as  for  instance  Fig.  262. 

on    the    coast    of    Maine,     Effect  of  hard  layers  (unshaded)  in  the  de- 
,  .   -       .  -        ,  nudation  of  mountains. 

Avhicli    IS    a    land    area 

partly  drowned  by  the  sea  (Fig.  211).  The  hard  rocks 
which  formed  hills,  now  exist  as  promontories,  capes,  or 
islands,  while  the  sites  of  the  softer  layers  are  occupied  by 
bays  or  straits. 

From  this  brief  statement,  it  is  seen  that  the  causes  for 
topographic  irregularities  are  most  complex.  They  are  to 
be  found  in  a  combination  of  internal  and  external  forces. 
The  land  is  in  movement,  and  the  forces  of  denudation  are 
at  work  carving  and  removing,  and  often  locally  build- 
ing. With  variations  in  altitude,  position,  and  kind  of 
rock,  many  complex  results  may  be  produced.  Above  all, 
it  must  be  borne  in  mind  that  these  changes  are  now  in 


406  PHYSICAL   GEOGRAPHY, 

progress ;  that  the  land  forms  are  still  changing ;  that  they 
have  been  different  in  the  past ;  and  that  the  future  will 
find  them  different  still.  Some  forms  have  reached  one 
stage,  and  some  another ;  but  all  are  developing  along  cer- 
tain lines  of  a  more  or  less  definite  nature,  notwithstanding 
the  fact  that  the  conditions  are  complex,  and  are  even 
undergoing  change  themselves.  Any  intelligent  study  of 
the  earth's  surface  must  be  made  with  these  facts  clearly  in 
mind. 


-•o*- 


REFERENCE  BOOKS. 

There  is  no  easily  accessible  book  in  which  the  relation  between  scenery 
and  geology  is  more  clearly  shown,  than  in  Geikie's  "Scenery  of  Scot- 
land."   Macmillan  &  Co.,  New  York.    Second  edition,  1887.     12mo.    $3.50. 

Powell.  — Physiographic  Features  (Natural  Geographic  Monographs, 
Vol.  I.,  No.  2).  American  Book  Co.,  New  York,  1895.  4to.  $0.20. 
(Some  suggestive  descriptions  of  the  origin  of  land  forms.) 


CHAPTER  XXII. 

MAN   AND   NATURE. 

General  Statement.  —  The  relation  between  man  and  the 
physical  conditions  of  the  earth's  surface  are  most  intimate, 
although  in  his  present  civilized  state  they  are  very  much 
less  important  than  in  the  past.  Formerly,  even  slight  bar- 
riers were  almost  impassable,  while  now  we  cross  them  with 
ease.  Less  than  a  half -century  ago,  the  journey  from  the 
Mississippi  to  the  west  coast  was  of  the  most  dangerous  kind, 
while  now,  in  a  few  days,  we  pass  over  the  mountains  and 
plateaus  with  ease  and  comfort.  While,  with  the  advance 
of  civilization,  man  is  becoming  less  dependent  upon  nature, 
at  the  same  time  he  is  increasing  his  power  to  control  and 
modify  the  surrounding  conditions.  So  the  subject  of  the 
relation  between  man  and  nature  naturally  divides  itself  into 
two  parts,  (1)  the  influence  of  nature  upon  man,  which  is  of 
decreasing  importance,  and  (2)  the  influence  of  man  upon 
nature,  which  is  all  the  time  increasing.  These  subjects  can 
be  treated  only  very  briefly. 

Modifying  Influence  of  Man.  —  In  many  small  ways  man 
is  engaged  in  the  work  of  modifying  the  natural  conditions 
of  his  surroundings.  He  protects  himself  from  the  rigorous 
climates,  and  thus  makes  his  existence  possible  in  zones  where 
otherwise  he  could  not  dwell.  He  modifies  the  forces  of 
nature  so  that  they  become  his  servants.  The  winds,  the 
rivers,  and  even  the  tides  are  converted  into  forces  which 
serve  him.     He  confines  the  river  within  its  banks  and  pre- 

407 


408  PHYSICAL   GEOGBAPHY. 

vents  the  flood ;  and  he  turns  the  river  waters  from  their 
course  to  lead  them  where  he  wills.  Deserts  are  transformed 
to  fertile  gardens ;  swamps  are  made  dry ;  the  sea  is  excluded 
from  the  marshy  lands  of  the  coast  lines  ;^  and  almost  every- 
where we  find  evidence  that  man  is  at  work  in  modifying  the 
surface.  The  earth  is  pierced  with  mining  shafts  and  tun- 
nels ;  new  water  connections  are  made  by  canals  across  the 
narrow  isthmuses;  inland  towns  are  connected  with  the  sea;, 
and  seashore  towns  are  made  into  seaports  by  the  construc- 
tion of  artificial  harbors. 

Notwithstanding  the  importance  of  these  effects,  there  is 
no  influence  of  man  more  potent  than  that  which  he  exerts 
upon  the  life  of  animals  and  plants.  Many  species  are  being 
perpetuated  under  domestication,  and  much  is  being  done  to- 
ward their  modification.  New  fruits  are  constantly  being  pro- 
duced, and  in  this  respect  the  influence  of  man  is  very  im- 
portant. Man  is  doing  a  great  work  in  distributing  animals 
and  plants  over  regions  which  are  not  properly  their  homes. 
Sometimes  the  effect  is  beneficial,  but  very  often  it  is  most 
disastrous.  For  instance,  the  rabbit  introduced  into  Australia 
has  become  a  national  pest ;  and  the  English  sparrow  is  com- 
pletely overrunning  this  country.  Insect  pests  and  diseases 
are  also  spread,  and  these  attack  not  merely  man,  but  also 
the  plants  and  animals. 

However,  it  is  in  the  destruction  of  life  that  the  most 
baneful  influence  of  man  is  noticed.  Animals  of  nearly  all 
kinds,  particularly  some  of  the  largest,  are  disappearing 
before  his  advance.  Several  species  have  been  entirely  ex- 
terminated, and  some,  siich  as  the  bison,  which  was  formerly 
so  abundant,  have  been  so  reduced  in  numbers  that  they  are 
almost  exterminated.  By  the  destruction  of  birds,  the  num- 
ber of  insects  has  been  increased ;  and  so  both  directly  and 
1  It  is  estimated  that  one-tenth  of  Holland  is  land  reclaimed  from  the  sea. 


MAN  AND  NATURE. 


409 


indirectly  the  influence  of  man  in  this  direction  has  been 
harmful. 

Man  and  the  Forest.  —  Probably  the  most  important  single 
influence  of  man  comes  from  his  habit  of  destroying  the 
forest  (Fig.  263).  In  many  ways  the  forest  covering  is 
important.     It  protects  the  soil  from  being  Avashed  away, 


Fig.  2(33. 

A  part  of  the  Adirondack  forest.    (Copyrighted,  1888,  by  S.  R.  Stoddard,  Glens 

Falls,  N.Y.) 


and  when  it  is  removed,  and  the  soil  turned  by  the  plow, 
both  weathering  and  the  removal  of  the  loose  materials  are 
increased.  In  some  places,  notably  in  France,  the  mountain 
sides,  from  which  the  forests  have  been  stripped,  have  been 
transformed  to  barren  wastes  of  rock  because  of  the  re- 
moval of  the  soil  by  the  rain.     In  other  places,  the  soil  has 


410 


PHYSICAL   GEOGBAPHY. 


been  so  gullied  that  it  is  unfit  for  cultivation.  A  part  of 
Mississippi  has  been  transformed  to  a  barren  waste  of  clay, 
the  features  of  which  resemble  those  of  the  Bad  Lands  of 
South  Dakota  (Plate  21).  The  effect  of  the  absence  of 
forests  is  well  illustrated  in  the  arid  lands,  where  the  forest 
covering  is  absent  because  of  natural  climatic  conditions. 
Here  every  rain  gullies  the  land ;  and  on  the  steeply  sloping 
hillsides,  the  removal  of  the  soil  by  rain  and  wind  action  has 
exposed  the  bare  rock  (Figs.  90  and  121). 


Fig.  264. 
Deforesting  in  the  Adiroudaeks. 

The  forest  serves  to  prevent  excessive  river  floods ;  for  it 
protects  the  snows  from  rapid  melting,  and  prevents  the  rain 
from  readily  passing  away  in  the  streams.  The  mat  of  leaves 
and  moss,  the  forest  Utter,  serves  as  a  great  sponge  which 
holds  the  water.  This  is  important  in  many  ways :  for  it 
makes  the  stream  less  liable  to  violent  floods ;  it  furnishes 
a  constant  and  rather  steady  supply,  both  to  springs  and 
streams;  and  it  furnishes  moisture  to  the  air.  With  the 
removal  of  the  forest  covering,  the  rain  and  the  melting  snow 


MAN  AND  NATURE. 


411 


pass  rapidly  into  the  rivers,  and  thence  to  the  sea  (Fig.  264). 
At  times,  exceptional  floods  are  produced ;  and  then,  when 
these  have  passed  away,  the  river  rapidly  loses  in  size,  until 
it  may  perhaps  become  nearly,  if  not  quite  dry  (Fig.  124). 
The  greater  part  of  the  water  passes  through  the  river  i:-  a 
few  days.  Every  person  of  maturity  who  has  dwelt  by  the 
side  of  a  stream  heading  in  a  region  once  forested,  but  now 
bared  of  its  tree  covering,  will  bear  testimony  to  the  fact 


Fig.  265. 
Bare  rock  exposed  to  weathering  by  removal  of  the  forest,  Mt.  Desert,  Me. 


that  streams  which  were  formerly  moderate,  clear,  and  per- 
manent, are  now  transformed  to  trickling  streams,  which  at 
times  become  raging  torrents,  clouded  with  sediment. 

This  influence  of  man  is  very  disastrous.  It  not  merely 
causes  the  removal  of  soil  from  the  mountains  (Fig.  265), 
but  distributes  this  over  the  lowlands ;  and  in  some  places, 
farms  have  been  rendered  uninhabitable  by  the  deposit  of 
sediment  during  times  of  flood.  Besides  this,  the  floods 
themselves  are  very  destructive  both  to  life  and  property ; 


412  PHYSICAL   GEOGRAPHY. 

and,  with  the  removal  of  the  forest  covering,  they  are  becom- 
ing ever  more  destructive.  Mills  cannot  count  upon  the 
same  steady  water  supply  that  they  formerly  had ;  springs 
quickly  become  dry;  and  there  is  some  reason  for  believing 
that  the  removal  of  the  forest  also  affects  the  climate.  This 
latter  point  has  been  suspected ;  but  it  has  never  been 
proven  that  the  forest  makes  the  rainfall  more  uniform  or 
greater  in  quantity.  The  reasons  for  suspecting  this  forest 
influence  are  (1),  that  the  damp  winds,  when  coming  in  con- 
tact with  the  cool  forests,  are  made  to  give  up  their  moisture 
more  readily  than  elscAvhere  ;  and  (2)  that  by  holding  the 
water  in  the  litter  beneath  the  trees,  a  greater  opportunity 
for  evaporation  is  furnished  than  when  the  forest  is  removed. 
It  is  held  that,  as  a  result  of  this,  the  air  is  rendered  moist 
and  is  more  liable  to  give  up  its  moisture. 

These  influences  are  so  important,  that  one  of  the  needs  of 
the  present,  is  greater  care,  intelligence,  and  patriotism  in  the 
relation  of  man  to  the  forest.  The  conditions  need  to  be 
carefully  studied,  destruction  ought  to  be  checked  so  far  as 
possible,  and  the  damage  of  past  destruction  should  be 
repaired  in  every  possible  case.  The  state  and  national 
governments  are  in  some  cases  engaged  in  this  work ;  but  it 
is  possible  for  nearly  every  one  to  do  something  toAvard  it. 
Unless  something  is  done,  the  heritage  of  the  land  which  we 
have  received,  will  not  be  transmitted  to  our  descendants  in 
so  good  a  condition  as  it  is  our  duty  to  leave  it. 

Influence  of  Nature  upon  Man It  is  quite  impossible  at 

present  to  estimate  the  effect  of  nature  upon  man;  for  in 
most  respects  we  have  risen  above  its  immediate  and  most 
important  modifying  influences.  Without  serious  difficulty, 
we  cross  mountains  and  continents,  rivers,  lakes,  and  even 
oceans ;  and  in  a  few  weeks  we  may  pass  around  the  entire 
world.     Every  generation  sees  an  increase  in  the  independ- 


MAN  AND  NATUBE.  413 

ence  between  man  and  nature,  and  the  completeness  of  the 
conquest  of  the  latter. 

This  has  not  always  been  so,  and  many  of  man's  most 
marked  characteristics  have  had  their  origin  in,  or  have  been 
impressed  upon  him  by  his  environment.  Even  now  we  find 
a  marked  difference  between  the  miner,  the  ranchman,  and 
the  farmer ;  and,  except  in  the  most  general  way,  the  effect 
of  climate  upon  man's  condition  cannot  even  be  estimated. 
Both  extremes  of  heat  and  cold  introduce  habits  of  mind  and 
body  quite  the  reverse  from  the  lively  mental  and  physical 
activity  of  the  inhabitants  of  the  temperate  zone.  The  in- 
habitant of  the  Arctic  loses  vitality  because  of  the  unequal 
struggle;  and  where  no  severe  struggle  for  existence  is 
necessary,  the  enervating  influence  of  the  tropical  sun  also 
decreases  vitality.  Under  the  bracing  air  of  the  temperate 
latitudes,  and  with  the  necessity  for  preparation  for  the  win- 
ter, man's  physical  and  mental  powers  have  been  improved ; 
and  this  is  probably  the  most  potent  reason  for  the  very 
striking  fact,  that  the  most  important  development  of  the 
race  has  taken  place  in  these  regions  ;  and  why  to-day,  nearly 
every  nation  of  marked  importance  is  situated  within  the 
temperate  belt,  and  mostly  near  the  arctic  limit  of  it. 

If  we  glance  back  to  the  time  when  man  was  less  inde- 
pendent of  nature,  when  his  railway  trains  were  not  present 
to  transport  him  across  river  and  mountain,  nor  his  steam- 
ships ready  to  bear  him  across  oceans,  we  find  a  very  close 
relation  between  the  life  of  men  and  nations  and  the  sur- 
rounding physical  conditions.  When  a  people  migrated  to 
a  new  land,  they  often  found  conditions  favorable  to  a  rapid 
development ;  and  if  they  were  sufiiciently  enclosed  and 
isolated  for  protection  from  invasion,  they  often  developed  to 
a  high  state.  However,  in  time  the  very  isolation  caused 
degeneration ;  and  we  see  this  illustrated  in  the  history  of 


414  PHYSICAL   GEOGRAPHY. 

such  people  as  the  Chinese  and  the  Egyptians.  Because  of 
the  climatic  conditions  under  which  they  lived,  some  of  the 
early  people  became  nomadic,  others  developed  into  agri- 
cultural nations,  and  still  others  into  seafaring  races. 

With  the  growth  of  commerce,  the  most  rapid  progress 
occurred  in  the  nations  most  favorably  situated  for  its  devel- 
opment. Thus  Italy,  nearly  isolated  from  other  countries, 
became  an  important  center  of  commerce.  Fresh  blood  and 
new  ideas  were  constantly  introduced,  and  gradually  the 
power  of  the  nation  increased  and  extended,  until  decay  came 
mainly  as  a  result  of  the  rapid  development,  and  the  nation 
crumbled  because  of  its  very  success.  In  ancient  Greece  we 
find  another  instance  of  the  influence  of  surrounding  condi- 
tions. The  very  rugged  topography  favored  independence  : 
for  even  in  small  areas,  different  states  could  exist  independ- 
ently ;  but  because  of  their  very  smallness,  they  were  com- 
pelled to  unite  against  common  foes. 

The  Mediterranean  was  the  seat  of  early  development; 
and  this  was  made  possible  by  reason  of  the  short  distances 
between  countries,  the  possibility  of  navigation  in  the  enclosed 
sea,  and  the  interchange  of  materials  and  ideas.  Here  the 
people  learned  lessons  in  navigation  which  made  the  explora- 
tion of  the  Atlantic  less  hazardous  ;  and  then,  by  land  and  by 
sea,  the  peoples  from  the  shores  of  the  Mediterranean  taught 
lessons  to  the  more  northern  races,  which,  when  well  learned, 
made  their  ultimate  success  possible  ;  and  then  the  students 
themselves  became  leaders.  The  shores  of  the  Mediterranean 
were  the  great  training  schools,  in  which  were  learned  most 
of  the  fundamental  ideas  upon  which  the  progress  of  the 
human  race  has  depended ;  and  even  now  its  influence  is 
felt  most  markedly  in  all  the  nations  of  the  world. 

Perhaps  there  are  no  better  illustrations  of  the  influence 
of  surroundings  upon  the  development  of  nations,  than  those 


MAN  AND  NATURE.  415 

furnished  by  Scandinavia  and  England.  The  roving  North- 
men, born  on  a  rocky  coast,  which  was  deeply  indented  with 
fjords,  made  the  sea  a  second  home.  Instead  of  farming,  they 
fished ;  instead  of  remaining  to  develop  their  inhospitable 
coast,  they  roved  the  seas  and  invaded  their  neighbors. 
With  that  hardiness  born  of  the  sea,  they  roamed  not  only 
along  the  European  shore,  but  sought  and  discovered  new 
lands  in  the  west.  They  not  only  learned  much  themselves, 
but  taught  much  to  others ;  and  the  lessons  that  they 
learned  and  imparted  were  mainly  due  to  their  neighbor- 
hood to  the  sea. 

In  England,  we  find  a  most  remarkable  illustration  of  the 
influence  of  environment.  The  climate  gave  vigor  to  the 
people ;  and  the  mixture  of  races,  that  had  come  in  earlier 
days,  made  a  nation  of  men  with  great  mental  and  physical 
power.  The  mineral  wealth  did  much  to  make  the  subse- 
quent development  possible,  for  it  became  sought  after  by 
many  nations.  Because  of  the  insular  condition,  this  store 
of  wealth  was  protected  without  great  difficulty  ;  and  yet  the 
islands  were  readily  visited  for  purposes  of  friendly  com- 
merce, and  the  stores  of  wealth  were  distributed  over  the 
world  to  the  profit  of  the  people  of  the  islands.  A  com- 
merce was  readily  developed ;  and  largely  upon  the  basis  of 
this,  England  became  what  she  is  to-day,  —  the  great  naval 
power  of  the  world,  and  the  possessor  of  colonies  in  every 
part  of  the  earth.  It  never  can  be  told  how  important  an 
event  it  was  in  the  development  of  nations,  when,  in  some 
prehistoric  time,  the  sea  first  passed  through  the  English 
Channel,  and  separated  the  British  Isles  from  the  mainland. 
With  land  connection,  the  history  of  Europe  and  the  world 
might  have  been  quite  different. 

When  we  look  at  the  maps  of  Europe  and  America,  two 
differences  of  a  most  striking  nature  attract  our  attention. 


416  PHYSICAL   GEOGBAPRY, 

The  one  is  the  extreme  irregularity  of  the  European  coast 
line,  the  other  the  great  number  of  nations  in  that  land. 
The  latter  fact  depends  upon  several  causes.  The  very- 
irregularity  of  the  coast,  and  the  great  diversity  of  the 
topography,  have  made  possible  the  development  of  distinct 
nations.  As  the  race  was  progressing,  mountain  barriers, 
and  even  rivers,  served  as  boundary  lines  between  separate 
tribes ;  and  some  of  these  are  preserved  to  this  day.  We 
find  Switzerland  completely  enclosed  between  other  nations, 
because  no  ancient  tribes  could  drive  these  people  from  their 
mountain  fortress.  To  fully  appreciate  the  importance  of 
these  influences,  one  needs  but  examine  a  physical  map  of 
Europe,  and  notice  how  the  mountains  and  the  seas  almost 
universally  serve  as  boundaries,  and  how  upon  every  penin- 
sula, there  is  one,  or  more,  independent  nation.  This  is  not 
so  in  America,  partly  because  the  conditions  are  not  so  diverse, 
but  chiefly  because  the  settlement  of  America  was  made  by 
races  which  had  already  developed. 

With  the  development  of  knowledge  and  poAver,  there  came 
an  era  of  exploration,  in  the  course  of  which  America  was 
discovered.  Even  this  discovery  depended  upon  peculiar 
physical  conditions  ;  for  had  Columbus  undertaken  to  make 
his  voyage  either  to  the  north  or  south  of  the  trade-wind 
belt,  the  chances  are  that  he  would  never  have  succeeded. 
With  the  favorable  trades  furnishing  fair  winds,  the  journey 
was  a  relatively  easy  one. 

The  explorers  and  settlers  found  the  American  land  occu- 
pied by  nomadic  races,  whose  power  of  resistance  to  invasion 
was  not  equal  to  the  skill  of  the  invaders  ;  and  with  the 
discovery  of  America  there  began  a  new  era.  Navigation 
increased ;  and  Spain,  who  had  learned  her  lesson  from  Italy, 
and  who  was  important  in  maritime  affairs  because  of  her 
extensive  coast  line,  became  a  powerful  nation.     As  in  Italy, 


MAN  AND  NATURE.  417 

success  caused  almost  utter  collapse,  and  Spain  lost  more 
than  she  gained. 

In  America,  the  invigorating  climate,  the  necessity  of  work, 
and  the  great  possibilities,  developed  a  race  which  has  become 
renowned  for  its  vigor  and  energy.  At  first  the  Appalachian 
barrier,  with  its  almost  impassable  forests,  prevented  entrance 
to  the  central  regions.  Therefore,  of  necessity,  settlements 
were  made  close  by  the  coast ;  and  it  is  said  that  in  1700,  one 
CO  aid  go  by  stage  from  Portland,  Maine,  to  Virginia,  spend- 
ing every  night  in  a  good-sized  village,  while  to  the  west  there 
was  an  impassable  wilderness  traversable  only  on  foot,  along 
the  Indian  trails.  The  large  waterways  leading  into  the 
interior  were  guarded,  the  Mississippi  and  the  St.  Lawrence 
by  other  nations,  and  the  Mohawk  by  a  powerful  Indian 
population. 

This  forest  barrier  caused  a  concentration  of  population, 
upon  which  much  of  the  success  of  our  later  development 
has  dependedo  It  determined  the  location  of  most  of  the 
great  centers  of  population ;  and  it  protected  the  English 
until  their  strength  had  sufficiently  increased  to  admit  of 
pushing  into  the  western  region,  and  displacing  the  unfor- 
tunate savage  occupants.  The  success  of  the  Revolution  also 
in  great  measure  depended  upon  the  concentration  of  pop- 
ulation thus  induced.  Had  the  people  been  less  connected, 
they  could  not  have  cooperated  so  well  as  they  did. 

When,  finally,  a  definite  roadway  was  established  across 
the  Appalachians,  which  was  first  done  over  Cumberland  Gap 
in  Tennessee,  the  most  difficult  step  in  the  western  progress 
was  taken.  The  great  treeless  prairies  were  then  reached, 
and  upon  these  agriculture  was  easily  pursued,  while  further 
progress  was  not  difficult ;  and  hence  the  Mississippi  valley 
became  speedily  developed.  When  once  the  way  was  found, 
other  openings  were  soon  made  across  the  forest  barrier. 
2e 


418  PHYSICAL   GEOGBAPHT. 

Then  came  the  discovery  of  the  wonderful  mineral  wealth 
of  the  west ;  and  the  eagerness  to  obtain  some  of  these  stores 
from  the  bosom  of  the  earth,  caused  an  almost  magical  de- 
velopment of  this  great  realm.  Cities  sprang  up  among  the 
mountains,  farms  were  developed  on  the  desert,  railroads 
crossed  the  mountain  chains,  states  grew  out  of  hitherto  un- 
settled territories ;  and,  in  a  quarter  of  a  century,  a  great 
region  was  transformed  from  an  unknown  waste,  inhabited 
only  by  savages,  to  the  most  remarkable  mineral-producing 
region  of  the  world.  Such  progress  as  this  could  be  made 
only  after  man  had  so  far  developed  as  to  be  able  to  defy 
and  overcome  the  most  formidable  of  obstacles. 

In  this  country,  the  influence  of  topography  upon  man  is 
seen  in  many  small  ways.  In  New  England,  particularly  in 
central  Massachusetts,  the  old  interior  towns  were  on  the  hills, 
which  were  fortresses  where  the  people  were,  in  a  measure, 
safe  from  Indian  attack;  and  even  now  we  find  many  of 
these  hilltop  villages,  which  at  present  are  scarcely  more 
than  relics  of  a  past  stage  in  development.  With  the  devel- 
opment of  the  industries,  manufacturing  determined  the  posi- 
tion of  the  more  important  interior  towns ;  and  these  were 
naturally  placed  in  the  valleys  which  afforded  a  good  supply 
of  water  power. 

Hilly  New  England  became  a  manufacturing  region ;  the 
states  of  the  level  and  fertile  prairie  formed  an  agricultural 
district ;  the  drier  plains  and  plateaus  of  the  west  became  the 
seat  of  the  cattle  industry ;  and  the  mountainous  region  of 
the  far  west  developed  into  a  mining  territory.  Many  of 
the  larger  cities  were  situated  on  the  seacoast,  because  here 
communication  and  commerce  with  other  countries  were  pos- 
sible. Even  the  sites  of  these  large  cities  were  determined 
by  the  form  of  the  coast  line ;  and  everywhere  that  we  may 
go  in  the  world,  we  find  an  almost  universal  relation  between 


MAN  AND  NATURE.  419 

man's  condition  and  his  surroundings.  The  delta  lands  are 
farming  districts,  the  semi-arid  plains  and  plateaus  are 
devoted  to  cattle  raising,  etc.;  but  while  man  is  largely  a 
creature  of  his  environment,  he  is  much  less  so  now  than 
ever  before ;  and,  little  by  little,  he  is  rising  above  the 
necessity  of  direct  dependence  upon  the  surrounding  physical 
conditions.  Formerly  he  was  guided  by  nature,  but  now,  in 
many  respects,  he  governs  and  guides  nature  to  suit  his  needs. 


-♦o«- 


REFERENCE   BOOKS. 

Shaler.  —  Nature   and   Man    in   America.     Scribner,   New  York,    1891. 
12mo.     $1.50. 

Guyot.  —  The    Earth    and    Man.      [Translated    by    Felton.J      Scribner, 
New  York.    Revised  edition,  1893.     12mo.    $1.75. 

Marsh.  — The  Earth  as  Modified  by  Human  Action.    Scribner,  New  York, 
1885.     8vo.     $3.50. 


CHAPTER  XXIII. 

ECONOMIC  PRODUCTS  OF  THE  EARTH. 

Soil.  —  The  crust  of  the  earth  furnishes  to  man  most  of 
the  material  which  he  needs  for  life  and  comfort.  The 
rocks  crumble  to  form  soil,  and  upon  this  exist  the  plants 
which  furnish  us  directly  or  indirectly  with  most  of  our  food 
supply.  In  this  the  trees  grow,  and  all  of  the  animals  of 
the  land  depend  upon  the  plant  life  which  exists  by  virtue 
of  this  soil  covering.  This  is  by  far  the  most  important 
mineral  product  of  the  earth,  for  upon  it  depends  our  exist- 
ence as  inhabitants  of  the  land. 

Building  Stones.  —  Within  the  earth,  as  a  part  of  the 
crust,  there  are  many  substances  which  man  finds  it  possible 
and  profitable  to  remove  for  his  own  use.  For  instance, 
there  are  the  building  stones,  of  which  we  have  many  kinds. 
The  great  masses  of  molten  rock,  which  have  been  intruded 
into  the  earth's  crust  from  below,  and  then  cooled,  and  finally 
reached  by  denudation,  furnish  us  with  great  quantities  of 
granite^  which  is  such  excellent  building  stone,  both  with 
regard  to  durability  and  appearance.  Sometimes  other 
forms  of  igneous  rocks  are  employed  for  building  pur- 
poses ;  and  among  these  we  find  great  variety  both  in 
color  and  texture. 

Granite  is  imitated  among  the  metamorphic  rocks,  where 
as  a  result  of  the  process  of  alteration,  a  structure  closely 
resembling  that  of  granite  is  sometimes  introduced  into  the 
gneissic  layers.     Indeed,  many  gneisses  are  sold  as  granites, 

420 


ECONOMIC  PBOnUCTS   OF  THE  EABTH.  421 

and  their  resemblance  is  often  so  close  that  one  can  tell  the 
difference  only  by  a  slight  banding  which  characterizes 
gneisses,  but  is  not  usually  present  in  granites. 

There  are  other  metamorphic  building  stones,  chiefly  slate 
and  marble.  Slate  represents  a  clay  rock  formed  as  a  deposit 
in  water,  and  then  subjected  to  heat  and  pressure,  so  that  its 
peculiar  cleavage  is  introduced.  Marble  is  the  metamor- 
phosed product  of  limestone,  in  which  the  carbonate  of  lime 
has  in  some  cases  been  transformed  to  crystals  of  calcite, 
causing  the  white  sugary  marble,  such  as  that  found  in  Ver- 
mont. In  other  cases  no  crystals  are  produced,  but  a  re- 
markable and  often  very  beautiful  banding  is  introduced. 
The  causes  for  this  metamorphism  are  usually  a  combination 
of  heat,  pressure,  and  motion  during  the  folding  of  the  rocks. 

The  sedimentary  rocks  themselves  also  furnish  us  with 
much  building  stone,  chiefly  in  the  form  of  sandstone  and 
limestone.  Among  these  there  is  great  variety,  both  as  re- 
gards texture  and  color;  and  this  class  of  building  stone 
is  extremely  common.  Indeed,  these  rocks  are  so  abundant 
that  only  the  best  can  be  extensively  used ;  and  in  many 
places  a  stone  is  quarried  for  home  use,  but  is  never  trans- 
ported far  beyond  the  quarry.  Few  stones,  and  these  mainly 
ornamental,  will  pay  for  transportation  to  great  distances, 
for  there  is  an  abundance  of  stone  for  ordinary  purposes, 
and  nearly  every  place  has  its  quarry. 

From  the  unconsolidated  clays  and  sands,  we  obtain  much 
material  for  building  purposes.  The  sand  for  plaster,  the 
clay  materials  for  some  cements,  and  the  clay  for  bricks,  are 
among  the  most  important  of  building  materials,  and  their 
sources  are  varied.  Some  are  decayed  rocks,  others  ocean 
deposits,  others  have  been  formed  by  rivers  or  lakes,  and 
many,  particularly  in  northern  United  States,  have  been 
brought  to  their  present  position  by  glacial  action. 


422  PHYSICAL   GEOGRAPHY, 

Economic  Deposits  of  Sedimentary  Origin.  —  Aside  from 
the  sedimentary  building  stones,  and  some  of  the  ores,  the 
crust  of  the  earth  contains  numerous  valuable  deposits 
formed  in  water.  Some  of  the  sandy  rocks  are  sufficiently 
rough  to  be  used  for  grinding  purposes ;  and  the  tiny  shells 
of  silica,  which  are  left  in  fresh-water  swamps  and  ponds  by 
certain  low  forms  of  animals  and  plants  (Infusoria  and 
Diatoms),  furnish  a  white  polishing  powder. 

When  lakes  have  their  outlets  cut  off,  and  evaporation 
exceeds  the  supply  of  water,  they  gradually  become  salt; 
and  finally  they  may  become  so  concentrated  that  some  is 
deposited  in  the  bottom  of  the  lake,  and  then  there  is  formed 
a  layer  of  rock  salt.  These  layers  may  be  buried  beneath 
other  strata,  and  at  some  later  time  be  discovered  as  a  salt 
mine.  In  the  Great  Basin  there  are  many  beds  of  this  kind, 
now  exposed  at  the  surface,  where  in  some  recent  times  a 
salt  lake  has  completely  dried  up ;  and  the  ranchmen  visit 
these  beds  with  wagons,  and  shovel  up  from  the  surface  all 
the  salt  that  thev  need. 

At  times  there  are  other  materials  deposited  with  this 
precipitated  salt.  In  this  way  such  substances  as  bromine, 
borax,  natural  soda,  and  even  gypsum,  which  is  used  as  a 
basis  for  plaster  of  paris,  are  deposited  in  layers. 

Left  to  itself,  the  soil  furnishes  the  plants  as  much  food  as 
they  require ;  but  when  man  interferes  and  tries  to  draw 
more  than  this  from  the  soil,  in  the  course  of  time  he  ex- 
hausts much  of  the  supply  of  plant  food,  and  it  is  then 
necessary  either  to  abandon  the  land  until  it  can  recover, 
or  else  to  artificially  supply  the  needed  substances.  For 
this  reason  fertilizers  of  one  kind  or  another  are  added. 
Sometimes  the  fertilizer  is  only  a  limestone,  or  it  may  be  a 
marly  clay  in  which  there  are  many  fossil  shells,  or  it  may 
be  one  of  the  natural  phosphates.     Phosphatic  materials  are 


ECONOMIC  PRODUCTS  OF  THE  EARTH.  423 

among  the  substances  needed  by  plants,  and  phosphate  is 
present  in  the  bones  of  many  mammals.  In  some  places,  as 
for  instance  in  South  Carolina,  near  Charleston,  and  in  many 
parts  of  Florida,  there  are  beds  of  a  phosphatic  rock  which 
owes  its  peculiar  character  to  the  presence  of  large  numbers 
of  bone  fragments.  These  are  great  mammalian  burial 
grounds,  and  man  is  now  drawing  upon  them  with  profit. 

Miscellaneous  Substances. —  There  are  many  other  products 
of  the  earth,  which  though  valuable,  are  of  minor  importance. 
Springs  containing  mineral  matter  in  solution  often  have 
medicinal  properties,  and  this  ensures  a  wide  sale  for  these 
mineral  waters.  Artesian  wells  (page  229)  are  of  no  little 
importance.  Sulphur  is  often  found  near  volcanoes  ;  graph- 
ite, which  occurs  in  metamorphic  rocks,  furnishes  the  black 
lead  for  our  pencils;  mica,  asbestos,  etc.,  are  also  found  in 
the  metamorphic  rocks ;  valuable  mineral  paints  are  usually 
colored  earths  due  to  rock  decay ;  and  to  these  many  other 
minor  products  might  be  added. 

Coal.  —  Seams  or  beds  of  coal  are  often  found  between 
layers  of  sandstone  and  limestone.  These  enclosing  rocks 
bear  evidence  of  having  been  formed  in  water,  and  the  fos- 
sils which  they  contain  often  prove  that  they  were  deposited 
in  salt  water.  Yet  the  coal  is  composed  of  the  remains  of 
land  plants,  and  even  tree  trunks  are  sometimes  found  pre- 
served in  the  beds.  In  some  cases  these  fossil  tree  trunks 
stand  upright,  with  their  roots  in  the  clay  beneath,  showing 
that  the  coal  bed  is  near  the  place  where  the  plants  grew. 
It  is  further  evident  that  they  were  then  covered  by  the 
sea,  and  that  in  this  the  marine  sediment  was  deposited. 
Often  there  are  several  beds  one  above  another,  each  prov- 
ing some  such  change  as  this. 

Much  about  the  origin  of  coal  cannot  be  considered  to  be 
finally  settled  ;  and  there  are  many  theories  for  its  origin. 


424  PHYSICAL   GEOGRAPHY, 

Since  we  cannot  enter  into  a  discussion  of  these,  it  will  be 
necessary  to  confine  ourselves  to  a  statement  of  what  seems 
to  the  author  to  be  the  most  probable  explanation.  Without 
doubt,  different  coal  beds  have  had  a  very  different  history. 
Some  represent  the  drifted  fragments  of  wood  that  have 
been  deposited  in  an  ancient  bay  or  estuary,  and  then 
buried  beneath  marine  deposits.  Thus  if  the  Mississippi 
delta  should  be  consolidated  into  rock  and  be  elevated,  there 
would  be  coal  seams  formed  where  rafts  of  logs  have  been 
stranded. 

There  also  seems  to  be  no  doubt  that  some  coal  beds  are 
nothing  more  than  swamps  which  were  formed  either  on 
shores  of  lakes,  or  as  the  last  stage  in  their  disappearance,  — 
in  a  measure  being  like  peat  bogs  consolidated  to  mineral 
fuel.  In  the  southern  part  of  Florida  there  are  a  great  num- 
ber of  swamps,  and  swampy  lakes,  in  which  there  is  a  vegeta- 
ble accumulation  several  feet  in  depth.  This  muck  is  made 
almost  entirely  of  plant  remains  with  practically  no  clay 
impurities.  If  this  low,  swampy  land  were  to  be  lowered 
beneath  the  sea,  these  beds  of  vegetable  matter  would  be 
covered  with  sediment,  and  a  coal  bed  would  be  begun. 
Later  the  same  conditions  might  be  repeated  and  another 
bed  be  formed,  etc. 

Even  at  present,  some  trees  (the  mangrove.  Fig.  205)  grow 
in  salt  water;  and  in  the  early  geological  ages  many  others 
probably  had  this  habit,  for  the  land  vegetation  of  these  early 
times  was  evolved  from  marine  plants.  At  this  time  there 
were  probably  great  salt-water  swamps,  in  which  many  of  the 
coal  beds  were  formed.  Very  likely  each  of  these  theories 
accounts  for  some  of  the  beds. 

The  coal  is  a  mineralized  form  of  vegetation,  produced  by 
a  slow  change,  in  the  course  of  which  many  of  the  volatile 
gases  have  been  driven  off.     There  is  every  gradation  from 


ECONOMIC  PRODUCTS  OF  THE  EARTH.  425 

wood  to  peat,  from  this  to  lignite  or  brown  coal,  then  to 
bituminous,  next  to  anthracite,  and  finally  even  to  graphite. 
This  does  not  require  great  heat,  but  slow,  steady  change. 
The  ash  of  the  coal  is  an  impurity,  often  bits  of  clay  and  sand 
that  were  deposited  with  the  coal. 

It  was  once  supposed  that  coal  was  formed  only  at  one 
period  in  the  history  of  the  earth,  and  this  was  given  the 
name  Carboniferous;  but  with  the  exploration  of  the  Cor- 
dilleras, this  has  been  shown  to  be  a  wrong  idea.  Workable 
beds  of  coal  were  not  formed  before  the  Carboniferous  time, 
because  in  those  early  ages  there  was  not  enough  land  vege- 
tation; but  ever  since  this  time,  coal  has  been  formed  where- 
ever  the  conditions  have  been  favorable.  In  the  west  there 
are  vast  quantities  of  Cretaceous  and  Tertiary  coal.  Indeed, 
in  such  places  as  the  swamps  of  Florida,  the  Dismal  Swamp, 
and  the  peat  bogs  of  the  north,  it  is  quite  probable  that  we 
are  even  now  witnessing  the  first  stages  in  coal  accumu- 
lation. 

Natural  Gas  and  Petroleum.  —  In  some  places,  wells  drilled 
into  the  sedimentary  rocks  reach  layers  containing  either  a 
natural  illuminating  gas,  or  petroleum.  These  products  are 
very  useful,  the  gas  for  fuel  and  light  near  the  wells,  the  oil 
for  the  basis  of  kerosene,  and  numerous  other  products. 
These  substances  occur  rather  irregularly;  and  wells  upon 
neighboring  farms  may  in  the  one  case  find  oil,  while  this  is 
not  discovered  in  the  neighboring  well.  However,  certain 
layers  are  liable  to  be  oil  bearing,  while  others  are  never 
known  to  contain  oil  or  gas.  After  awhile  both  the  oil 
and  gas  wells  gradually  decrease  in  volume,  and  must  finally 
be  abandoned.  Therefore  the  supply  is  not  constantly  fur- 
nished at  as  rapid  a  rate  as  the  drain. 

These  substances  are  the  product  of  a  slow  natural  distil- 
lation of  the  organic  remains  of  the  rocks;  and  they  quite 


426  PHYSICAL   GEOGBAPHY, 

closely  resemble  substances  which  we  produce  artificially. 
The  oil  is  not  markedly  different  from  that  produced  from 
fish  refuse ;  and  the  gas  resembles  the  illuminating  gas 
caused  by  burning  coal.  The  change  is  a  slow  one,  and 
in  the  course  of  time,  enough  accumulates  in  a  certain 
layer  to  make  a  gas  or  oil  deposit.  In  some  cases  this 
accumulation  is  in  the  same  layer  in  which  the  distillation 
took  place  ,*  in  others,  the  substances  have  migrated  into  a 
neighboring  layer.  Like  water,  they  are  able  to  slowly 
seep  through  the  rocks ;  and  in  their  passage,  they  may 
come  into  a  coarse  sandy  rock,  and  be  imprisoned  there 
by  an  overlying  clay  layer  which  is  too  impervious  for  easy 
passage.  In  these  cases  there  is  a  resemblance  to  the  con- 
ditions favoring  artesian  wells.  This  is  the  common  case 
in  the  Pennsylvania  wells ;  but  in  Indiana  these  substances 
occur  in  a  limestone. 

Ore  Deposits.  —  Some  of  the  metals  which  occur  in  the 
earth  possess  qualities  which  make  them  useful  to  man;  and, 
as  we  know,  great  effort  is  made  to  obtain  them.  Iron,  gold, 
silver,  copper,  etc.,  serve  us  in  many  ways.  In  the  earth 
they  generally  occur  in  association  with  other  elements,  in 
the  form  of  minerals ;  and  when  mined,  these  have  first  to  be 
separated  from  their  companion  minerals,  with  which  they 
are  mechanically  mixed ;  and  then  it  is  usually  necessary  to 
separate  the  metal  from  the  elements  with  which  it  is  chemi- 
cally combined.  Therefore  in  obtaining  these  substances 
from  the  earth,  many  complex  and  often  very  costly  methods 
are  employed.  In  order  that  this  may  be  profitable,  the  ores 
of  the  metals  must  occur  in  a  somewhat  concentrated  condi- 
tion, and  they  must  be  in  a  place  from  which  they  may  be 
obtained  without  too  great  expense.  Thus  a  copper  mine 
that  would  pay  in  New  England  or  New  York,  might  not  be 
profitable  if  situated  among  some  of  the  nearly  inaccessible 


ECONOMIC  PRODUCTS   OF  THE  EARTH.  427 

mountains  of  the  west.  Where  the  deposit  is  very  rich,  it  is 
often  profitable  to  tunnel  into  the  earth  to  a  depth  of  several 
thousand  feet. 

Ores  occur  in  the  rocks  of  the  crust  under  many  different 
conditions.  Sometimes  the  ore  is  a  native  metal,  as  is  the 
case  with  most  of  the  gold  which  is  mined ;  but  more 
commonly  it  is  a  simple  compound  of  a  metal.  It  would 
be  quite  impossible  to  state  in  a  few  words  the  various 
ways  in  which  the  ores  occur,  and  only  one  or  two  of  the 
most  common  kinds  can  be  described,  and  these  only  in  a 
general  way. 

Some  of  the  ores  have  been  deposited  in  beds,  by  a  process 
of  replacement.  That  is,  some  mineral  or  rock,  such  as 
quartz  or  limestone,  has  had  its  place  taken  by  the  ore, — 
this  being  deposited  bit  by  bit,  while  the  water  which  car- 
ried the  solution  took  away  an  equal  amount  of  the  original 
mineral.  This  resembles  the  replacement  of  wood  tissue  by 
silica  —  a  process  known  as  petref action.  Some  ore  deposits 
represent  the  mere  gathering  together  of  substances  into 
bunches,  known  as  concretions,  the  cause  for  the  accumu- 
lation being  still  unsolved. 

Much  more  commonly,  ores  are  deposited  in  some  cavity  in 
the  earth  ;  and  the  most  common  of  these  is  the  fissure  which 
accompanies  faulting.  In  this  break  in  the  strata,  which 
often  extends  to  great  depths,  ore  is  deposited  from  solution 
in  water.  This  underground  water  is  often  highly  heated, 
and  contains  in  solution  alkaline  or  acidic  substances  which 
give  to  it  great  power  of  dissolving  and  altering  min- 
erals. By  complex  chemical  reactions,  which  are  not  well 
understood,  these  ores  are  deposited  in  veins,  usually  in 
bands,  and  commonly  associated  with  other  minerals  which 
are  not  of  value.  Even  ores  of  gold  or  silver  are  frequently 
deposited  in  this  way. 


428  PHYSICAL   GEOGRAPHY, 

Another  important  way  in  which  ores  occur,  is  in  surface 
deposits  of  sedimentary  origin.  For  instance,  when  a  gold- 
bearing  rock  decays,  the  nearly  indestructible  gold  resists 
weathering  ;  and  being  a  heavy  substance,  as  it  is  being 
washed  down  toward  the  sea,  it  tends  to  accumulate  on  the 
stream  bottom,  forming  what  is  known  as  stream  or  placer 
gold  deposits.  This  is  the  condition  in  which  a  great  deal 
of  the  gold  of  the  w^orld  has  been  found  ;  and  this  precious 
metal  occurs  in  such  deposits  in  the  west,  in  Siberia,  Aus- 
tralia, and  many  other  places.  Both  tin  and  platinum  are 
also  found  in  a  similar  condition. 

Distribution  of  Ore  Deposits.  —  The  valuable  ore  deposits 
which  are  found  in  fissures,  are  not  present  in  all  parts  of  the 
crust ;  but  for  the  most  part  they  are  confined  to  mountainous 
regions.  The  Cordilleran  region  of  the  west  is  a  most  strik- 
ing illustration  of  this ;  for  these  mountains  form  the  most 
remarkable  mineral  district  of  the  world.  While  this  dis- 
trict produces  only  a  few  of  the  metals,  it  is  not  because  the 
others  (such  as  iron)  are  absent,  but  because  in  that  region 
the  conditions  are  too  unfavorable  for  the  extraction  and 
marketing  of  those  which  are  not  very  valuable. 

The  reasons  for  the  great  importance  of  the  Cordilleras  in 
the  production  of  metals,  are  mainly  tAvo.  In  the  first  place, 
among  these  mountains  there  are  many  faults,  and  other 
cavities,  in  which  ore  may  be  deposited.  There  are  also 
numerous  volcanic  rocks  of  recent  date,  a  point  of  consider- 
able importance.  The  heat  from  these  lava  intrusions  fur- 
nishes to  the  underground  water  a  temperature  sufficiently 
high  for  important  action.  Probably  even  at  present  some 
of  the  hot  springs  of  that  region  receive  their  heat  from 
buried  lava  intrusions  ;  and  probably  also,  mineral  deposits 
are  being  made  in  their  tubes  at  a  considerable  distance  from 
the  surface.     A  second  reason  why  the  presence  of  igneous 


ECONOMIC  PBODUCTS   OF  THE  EAETH,  429 

rocks  aids  in  ore  formation,  is  that  there  is  a  larger  percent- 
age of  metals  in  these  than  in  others.  Therefore  water 
which  is  percolating  through  and  altering  them,  finds  a 
greater  supply  of  metals  for  solution  than  would  be  the 
case  if  passing  through  most  sedimentary  rocks. 

Mineral  Wealth  of  the  United  States.  —  Mainly  because  of 
the  Cordilleras,  the  United  States  is  the  great  mineral 
country  of  the  world.  Of  the  following  metals  it  produces 
more  than  any  other  country :  gold,  silver,  iron,  and  copper, 
wdiich  are  the  most  important  of  metals  ;  and  in  the  pro- 
duction of  lead,  zinc,  and  mercury,  it  holds  second  rank. 
Its  output  of  coal  is  greater  than  that  of  any  other  nation 
excepting  Great  Britain,  while  no  other  country  supplies  so 
much  petroleum  and  natural  gas.  In  some  of  the  minor 
substances  it  also  holds  a  high  rank.  Indeed,  we  produce 
nearly  every  important  mineral  substance  found  in  the 
earth's  crust ;    and  usually  our  production  is  very  great. 

The  importance  of  the  mineral  industry  of  this  country,  is 
shown  by  the  fact  that  in  1892  the  mineral  production  was 
valued  at  nearly  1700,000,000,  of  which  about  1300,000,000 
came  from  the  metals,  —  mainly  iron,  silver,  gold,  copper, 
lead,  zinc,  and  mercury.  For  the  most  part  this  represents 
the  crude  product;  and  in  the  utilization  of  this  in  manu- 
facturing, there  are  industries  also  worth  many  hundred 
millions  of  dollars  :  so  that,  directly  and  indirectly,  the  min- 
eral industry  of  the  country  is  one  of  the  most  important. 

The  few  facts  that  follow,  will  serve  to  furnish  an  idea  of 
the  distribution  of  this  product.  According  to  the  census, 
the  leading  mineral  state  is  Pennsylvania,  which  produces 
more  coal,  petroleum,  gas,  and  stone,  than  any  other  state. 
In  1889  the  value  of  its  product  was  $150,000,000.  Second 
in  rank  is  Michigan,  which  produces  most  iron  and  salt,  and 
is  the  second  in  the  production  of   copper.     Then  comes 


430  PHYSICAL   GEOGBAPHT, 

Colorado,  which  leads  in  the  production  of  silver  and  lead, 
and  is  second  in  the  production  of  gold ;  and  Montana  fol- 
lows, leading  in  the  production  of  copper,  and  second  in  the 
output  of  silver.  The  east  excels  in  the  production  of  non- 
metallic  substances,  and  the  west  in  metals. 

This  astonishing  mineral  wealth  has,  in  no  small  degree, 
been  responsible  for  our  development  as  a  nation ;  and  there 
are  still  great  undiscovered  stores.  There  seems  to  be  almost 
no  limit  to  the  possibilities  in  this  direction,  and  our  Alaskan 
territory  promises  to  add  to  this  wealth.  Nature  has  been 
most  prodigal  in  lavishing  her  favors  upon  this  country,  for 
she  has  given  us  nearly  all  that  man  could  request :  great 
variety  of  climatic  conditions,  an  almost  infinite  variety  of 
topography,  a  soil  wonderfully  rich  over  a  great  area,  a  forest 
covering  from  which  we  have  been  able  to  draw  heavily  for 
over  a  century,  water  power  for  the  mills,  harbors  for  the 
commerce,  mineral  deposits  of  marvelous  wealth,  —  these 
are  things  which  mark  our  country  as  one  of  great  possi- 
bilities, and  which  have  made  possible  our  present  prosperity, 
and  upon  which  we  may  predict  so  much  for  the  future. 


REFERENCE  BOOKS. 

Kemp.  — The  Ore  Deposits  of  the  United  States.     Scientific  Publishing 

Co.,  New  York,  1893.     8vo.     84.00. 
Phillips.  —  Ore  Deposits.     Macmillan  &  Co.,  New  York,  1884.    8vo.     $7.50. 

Tarr.  —  Economic  Geology  of  the   United  States.    Macmillan  &  Co., 
New  York.    Second  edition  (revised),  1895.     8vo.     |3.50. 


APPENDIX   I. 

METEOROLOGICAL   INSTRUMENTS,    APPARATUS,  AND 

METHODS. 

By  instruments  we  are  able  to  measure  the  temperature,  pressure,  wind 
force  and  direction,  rate  of  evaporation,  percentage  of  moisture  in  the 
air,  amount  or  percentage  of  sunshine,  rainfall,  and  other  weather  phe- 
nomena. In  order  to  understand  these  instruments,  it  is  necessary  to 
handle  them  just  as  the  meteorological  observer  does.  Mere  descrip- 
tion can  serve  only  to  explain  the  principle  upon  which  they  depend. 

Thermometric  Records.  —  In  measuring  the  temperature,  use  is  made  of 
the  principle  that  certain  substances  expand  when  heated  and  contract 
when  cooled.  Ordinarily  it  is  more  convenient  to  employ  a  liquid,  and 
that  best  adapted  to  this  purpose  is  mercury.  However,,  where  tem- 
peratures below  the  freezing-point  of  mercury  are  liable  to  be  experi- 
enced, alcohol  is  used. 

The  thermometer  is  graduated  into  degrees  according  to  some  scale,  and 
different  scales  are  employed,  the  most  common  in  use  being  the  Fahr- 
enheit, which  is  adopted  in  nearly  all  English-speaking  countries,  and 
is  used  in  this  book.  The  two  points  of  importance  in  the  Fahrenheit 
scale  are  the  freezing-point,  which  is  placed  at  32°,  and  the  boiling- 
point,  which  is  placed  at  212°.  In  the  Centigrade  scale,  the  principle  is 
the  same ;  but  in  this  case  the  degrees  are  larger,  the  freezing-point  being 
placed  at  0°,  and  the  boiling-point  at  100°.  Therefore,  in  converting  the 
Fahrenheit  to  the  Centigrade  scale,  1°  of  Centigrade  is  equal  to  1.8°  of 
Fahrenheit,  and  to  this  must  be  added  32°.  All  are  familiar  with  ther- 
mometers, and  the  principle  upon  which  they  depend  is  easily  under- 
stood. 

Much  care  is  needed  in  the  construction  of  a  good  and  accurate  ther- 
mometer, and  there  are  some  cheap  and  very  inaccurate  instruments. 
This  is  one  reason  why  the  observations  of  temperature  made  by  different 

431 


432  PHYSICAL   GEOGBAPHY, 

people  may  vary  so  widely,  even  though  made  in  almost  the  same  location. 
Another  very  important  reason  for  this  difference  is  the  fact  that  the 
thermometer  is  not  always  wisely  placed.  In  order  to  obtain  a  true 
measure  of  the  temperature  of  the  air,  it  is  necessary  that  neither  the 
sun,  nor  any  warm  body  on  the  earth,  shall  influence  the  air  whose  tem- 
perature is  to  be  measured.  At  meteorological  stations,  the  thermometers 
are  placed  in  a  thermometer  shelter^  which  consists  of  a  frame,  open  so 
that  the  air  may  pass  through  it,  and  yet  sufficiently  closed  to  prevent 
the  sun's  rays  from  striking  upon  the  thermometer.  This  is  raised  about 
10  feet  from  the  surface,  and  is  placed  away  from  buildings. 

Of  late,  metallic  thermometers  have  come  into  use ;  and  these  depend 
upon  the  effect  of  heat  and  cold  on  metal  strips  or  springs  enclosed 
within  a  clock-like  case.  They  are  not  so  accurate  as  the  well-made 
mercurial  thermometers,  and  their  chief  value  is  in  obtaining  a  continu- 
ous record.  The  self-recording  thermometers,  or  thermographs,  are  mostly 
of  this  class.  As  the  metal  expands  or  contracts,  it  causes  an  index  hand 
to  move  back  and  forth  over  a  dial,  and  upon  this  index,  a  pen  or  pencil 
may  be  fixed  in  such  a  manner  as  to  press  against  a  sheet  of  paper.  As 
the  temperature  rises  and  falls,  the  needle  is  made  to  move  backward 
and  forward,  and  therefore  the  pen  is  also  moved  over  the  paper.  For 
the  purpose  of  obtaining  a  record  of  the  time  at  which  these  changes 
occur,  the  paper  itself  is  also  made  to  move  by  means  of  a  clock-work 
attachment ;  and  therefore  a  record  of  all  the  temperature  changes 
throughout  the  day,  may  be  automatically  registered. 

It  is  often  found  desirable  to  have  a  record  of  the  highest  and  lowest 
temperatures  of  the  day  made  by  a  mercurial  thermometer.  For  this 
purpose  the  maximum  and  minimum  thermometers  are  used,  which,  by  a 
special  contrivance,  record  the  very  highest  and  lowest  temperatures  of 
the  day,  but  do  not  give  any  record  of  the  time  at  which  these  occurred. 
The  thermometer  itself  gives  us  a  record  of  the  air  temperature,  which 
is  very  different  from  the  energy  which  comes  from  the  sun.  If  the 
bulb  of  a  thermometer  be  blackened  by  black  paint  or  lampblack,  and 
the  instrument  be  placed  in  the  direct  rays  of  the  sun,  it  is  found  that 
the  temperature  rises  very  much  higher  than  in  the  case  of  a  thermom- 
eter in  the  shade,  or  even  of  a  natural  thermometer  exposed  to  the  sun's 
rays.     Such  an  instrument  is  known  as  the  hlack-bulb  thermometer. 

Barometric  Records.  —  The  air  has  weight,  and  at  the  sea  level  this 
weight,  or  air  pressure,  averages  approximately  15  pounds  on  every 
square  inch.    The  air  pressure  at  any  given  place  is  liable  to  many  varia- 


APPENDIX  L  433 

tions,  and  it  is  the  purpose  of  the  barometer  to  detect  these  changes.  The 
principle  of  the  barometer  is  that  a  column  of  air  will  exactly  counter- 
balance a  column  of  equal  weight  of  any  liquid.  Thus  water  in  a 
vacuum  will  be  made  to  rise  to  a  height  of  about  32  feet.  In  other 
words,  it  counterbalances  the  pressure  of  the  air,  and  the  pump  is 
based  upon  this  principle,  water  being  forced  into  the  partial  vacuum 
caused  by  pumping.  We  could  use  a  column  of  water  for  a  barometer 
just  as  well  as  mercury,  which  is  ordinarily  used;  but  such  a  barometer, 
since  it  would  need  to  be  at  least  35  feet  in  height,  would  be  most 
unwieldy.  The  mercurial  barometer  consists  of  a  tube  of  glass,  sealed 
at  one  end  and  partly  filled  with  mercury.  Above  the  column  of  mer- 
cury is  a  practical  vacuum,  and  the  lower  part  of  the  tube  is  immersed 
in  a  cistern  of  mercury.  As  the  air  pressure  varies,  the  mercury  is  caused 
to  rise  in  the  tube,  or  to  descend  from  it  into  the  cistern ;  and  when  the 
air  is  heavy,  we  speak  of  a  high  barometer;  when  it  is  relatively  light,  of 
a  low  barometer.  In  meteorology,  these  terms  have  come  to  be  synony- 
mous with  high  pressure  and  low  pressure. 

The  tube  of  the  instrument  is  graduated  in  inches,  and  at  the  sea 
level  the  average  height  of  the  mercury  in  the  barometer  is  about  30 
inches.  The  method  of  reading  the  barometer,  and  the  use  of  the 
vernier  scale,  can  be  understood  only  by  handling  an  instrument. 

Several  forms  of  barograph  are  employed  to  convert  the  record  of  the 
change  in  pressure  into  graphic,  continuous  records.  The  rising  and 
falling  column  of  mercury  may  be  automatically  photographed,  or  the 
rise  and  fall  of  the  column  may  be  recorded  by  electricity;  but  most 
commonly  some  form  of  aneroid  barometer  is  employed.  The  aneroid 
depends  upon  the  effect  of  air  pressure  upon  a  metallic  diaphragm ;  and 
as  the  index  hand  moves  one  way  or  the  other,  it  carries  a  pen,  which 
marks  the  changes  upon  a  sheet  of  paper  revolving  on  a  cylinder,  just 
as  in  the  case  of  the  self-recording  thermometer. 

Measurement  of  Wind  Direction  and  Force.  —  To-day  the  direction  of 
the  wind  is  measured  in  very  nearly  the  same  manner  that  it  has  been 
for  centuries.  The  wind  vane  is  a  familiar  feature.  The  foi'ce  of  the 
wind,  or  its  velocity,  may  be  roughly  estimated  by  any  observer.  A  state- 
ment that  the  wind  velocity  is  40  miles  an  hour,  means  that  in  one  hour 
the  wind  travels  that  distance.  For  accurately  measuring  this  velocity, 
an  instrument  known  as  the  anemometer  is  used.  It  consists  of  four  cups, 
fixed  upon  a  cylinder,  which  are  revolved  by  the  wind  at  rates  depend- 
ing upon  its  velocity.  The  air  enters  these  cups  and  whirls  them  about, 
2f 


434  PHYSICAL   GEOGRAPHY. 

very  much  as  water  enters  a  turbine  wheel  and  causes  it  to  revolve.  By 
means  of  a  series  of  wheels,  each  revolution  of  the  anemometer  is  re- 
corded, and  this  may  be  transmitted  by  electricity  to  some  place  where 
an  automatic  record  is  kept  in  miles  per  hour. 

Measurement  of  Evaporation.  —  The  measurement  of  evaporation  is 
made  in  inches  of  water  evaporated  from  a  surface  exposed  to  the  air. 
Almost  any  dish  can  be  used,  and  the  scale  of  inches  be  marked  upon 
it ;  or  the  measurement  may  be  made  with  a  graduated  rule.  Since  the 
rate  of  evaporation  varies  with  the  temperature,  it  is  best  to  attempt  to 
imitate  natural  conditions  as  nearly  as  possible,  though  this  is  not  ordi- 
narily done.  The  best  way  is  to  place  the  evaporating  pan  in  a  quiet 
body  of  water,  allowing  it  to  float  on  the  surface.  There  are  various 
contrivances  for  obtaining  a  continuous  record. 

Measurement  of  Moisture  in  the  Air.  —  The  measure  of  the  relative 
humidity  is  often  obtained  by  the  hair  hygrometer,  which  is  a  bundle  of 
human  hair  from  which  the  oil  has  been  extracted.  As  the  amount  of 
moisture  in  the  air  increases,  the  hair  absorbs  more  and  more,  and  as  it 
does  so,  expands;  and,  since  one  end  is  fixed  while  the  other  moves 
freely,  this  expansion  may  be  made  to  record  itself  against  a  graduated 
glass  scale. 

The  best  method  is  that  of  the  use  of  the  sling psychrometer.  This  instru- 
ment consists  of  two  thermometers  fixed  side  by  side  upon  a  board.  One 
is  an  ordinary  thermometer,  the  other  has  a  piece  of  wet  muslin  placed 
around  its  bulb.  The  instrument  is  whirled  in  the  air,  and  the  water 
evaporates  from  the  wet  muslin,  the  rate  of  evaporation  varying  wuth  the 
humidity  of  the  air.  If  the  air  is  very  dry,  evaporation  takes  place 
rapidly ;  if  damp,  it  proceeds  with  slowness.  Since  evaporation  produces 
cold,  the  temperature  of  the  wet  bulb  thermometer  descends  lower  than 
that  of  the  ordinary  thermometer.  By  reading  these  two  records  of  tem- 
perature, the  relative  humidity  of  the  air  is  readily  determined  by  means 
of  a  series  of  tables  which  are  constructed  for  and  furnished  by  the 
Weather  Bureau  at  Washington.  The  relative  humidity  is  expressed  in 
per  cents  between  0,  which  is  perfectly  dry  air  (a  condition  which  never 
occurs),  and  100,  which  is  saturated  air.  From  this  measure  the  dew- 
point  may  also  be  determined. 

Study  of  Clouds  and  Sunshine.  —  Various  instruments  are  used  to 
obtain  a  record  of  the  amount  of  sunshine,  and  these  may  be  found 
described  in  the  books  referred  to  at  the  end  of  this  Appendix.  Much 
work  of  a  scientific  nature  is  also  being  done  in  the  study  of  clouds,  in- 


APPENDIX  I.  435 

eluding  the  measurement  of  height,  the  photographing  of  cloud  forms, 
etc.     We  cannot  devote  space  to  a  description  of  these. 

Measurement  of  Rainfall.  —  By  the  rain  gauge,  rainfall  is  measured  in 
inches,  an  inch  of  rainfall  being  an  actual  inch  of  water  which  has  fallen 
upon  the  surface.  This  is  a  cylinder  having  a  broad,  funnel-shaped  top, 
with  the  outlet  to  the  funnel  extending  into  an  inner  cylinder.  The 
water  falls  upon  the  surface  of  this  funnel,  and  runs  into  the  inner 
cylinder;  and  the  proportion  of  this  to  the  surface  of  the  funnel  is  as 
1  to  10.  By  this  means  the  actual  rainfall  is  magnified  10  times  in  the 
inner  cylindei-,  so  that  light  rainfalls  may  be  readily  measured. 

The  snowfall  is  often  measured  in  the  same  instrument ;  and  in  order 
to  express  the  snowfall  in  inches  of  rain,  as  is  usually  done,  the  snow 
that  is  collected  in  the  cylinder  is  melted.  About  one  inch  of  rain  is 
equal  to  10  inches  of  snow;  but  in  this  there  is  much  variation,  for 
some  snows  are  composed  of  very  compact  crystals,  while  others  are  light. 
In  some  cases  the  depth  of  the  snow  is  measured  and  divided  by  10,  in 
order  to  be  reduced  to  inches  of  rain.     This  is  roughly  correct. 

Self -registering  rain  gauges  are  made,  the  record  of  rainfall  being  kept 
either  by  means  of  a  float  that  rises  as  the  rainfall  increases,  or  else  by 
means  of  a  pair  of  scales  upon  which  the  rain  gauge  is  placed. 

Meteorological  Methods  and  Results.  —  At  present,  nearly  every  civil- 
ized nation  has  a  weather  bureau  from  which  are  issued  weather  maps 
and  predictions.  In  the  United  States  the  central  bureau  is  at  Wash- 
ington, and  many  of  the  states  have  similar  bureaus.  The  national 
bureau  issues  daily  maps  and  other  publications  describing  or  predicting 
the  weather. 

The  information  obtained  in  this  way  is  of  much  value.  The  pre- 
dictions of  the  Weather  Bureau  are  very  closely  followed  by  the  masters 
of  sailing  vessels,  and  much  loss  of  life  and  property  has  been  prevented 
by  this  means.  Predictions  of  excessively  cold  weather,  and  of  storms, 
give  much  information  concerning  the  weather  changes  that  are  liable 
to  occur;  and  by  means  of  the  warnings  farmers  are  sometimes  able  to 
prepare  against  unusually  early  or  late  frosts. 

For  the  purpose  of  obtaining  information  which  shall  serve  as  a  basis 
for  predictions,  the  Weather  Bureau  has  stations  distributed  over  various 
parts  of  the  country,  at  which  observers  read  the  records  of  the  several 
kinds  of  instruments.  These  observations  are  made  at  regular  times 
during  the  day,  and  the  results  are  telegraphed  to  central  stations, 
where  they  are  all  worked  over  and  plotted  upon  a  map.     Then,  with  the 


436  PHYSICAL   GEOGRAPHY. 

knowledge  of  the  changes  that  have  occurred  in  the  preceding  days, 
and  knowing  what  changes  are  liable  to  follow,  predictions  of  greater  or 
less  accuracy  are  made,  in  some  cases  for  several  days  in  advance.  In 
many  respects  these  predictions  are  of  great  importance  ;  but  in  addition 
to  this  result  of  the  work,  we  are  rapidly  obtaining  much  scientific  infor- 
mation concerning  the  air.  We  are  also  obtaining  many  facts  relating 
to  the  general  climatic  features  of  the  country,  and  of  the  world.  But 
in  these  directions  much  less  is  being  done  than  should  be;  for  until 
we  know  more  about  the  air  and  its  behavior,  we  may  not  expect  to 
obtain  more  accurate  predictions. 

Upon  a  weather  map  (Fig.  46)  the  wind  direction  is  plotted  in  the  form 
of  a  series  of  arrows  pointing  in  the  direction  toward  which  the  wind  is 
blowing.  The  temperature  is  also  placed  upon  them,  and  lines  of  equal 
temperature,  or  isotherms,  are  drawn  across  the  country.  The  pressure 
of  the  air  is  also  graphically  shown  on  the  maps  by  a  series  of  lines 
which  are  known  as  isobars,  or  lines  of  equal  barometic  pressure,  each 
tenth  of  an  inch  being  represented  by  an  isobar.  The  amount  of  rainfall 
at  the  different  stations  is  printed  on  the  maps.  Thus  at  a  glance  one 
may  see  the  weather  conditions  of  a  whole  country ;  and  by  studying  a 
series  of  these  maps  made  for  several  successive  days,  one  is  able  to  trace 
the  variations  in  weather  conditions  for  different  places. 


REFERENCE  BOOKS. 

Waldo.  —  Modern  Meteorology.     (Contemporary  Science  Series.)     Scrib- 

ner,  New  York,  1893.     12mo.     $1.25. 
Russell.  —  Meteorology.     Macmillan  &  Co.,  New  York,  1895.     8vo.     $4.00. 

Abbe.  —  Treatise  on  Meteorological  Apparatus  and  Methods,  Annual 
Keport  U.  S.  Signal  Service  for  1887.  Part  II.  Washington,  1888. 
There  is  also  a  description  of  instruments  in  the  first  part  of  the  Annual 
Report  of  the  Weather  Bureau,  1891-1892. 

For  obtaining  the  Dew-point  and  Relative  Humidity,  see  The  Tempera- 
ture OP  THE  Dew-point,  etc.,  U.  S.  Signal  Service,  1889. 

For  all  kinds  of  Meteorological  Tables,  see  Guyot,  Tables  :  Meteoro- 
logical AND  Physical.  Fourth  edition,  1884.  8vo.  Smithsonian  Miscel- 
laneous Collections,  Washington.     $3.50. 


APPENDIX  II. 

TOPOGRAPHIC    MAPS. 

The  study  of  the  land  is  greatly  facilitated  by  the  use  of  maps,  and 
for  this  reason  some  space  may  be  devoted  to  the  description  of  the 
more  common  kinds  of  topographic  maps.  By  far  the  best  means  of 
representing  land  irregularities  is  the  model  (Fig.  266),  upon  which  ele- 
vations are  shown  as  elevations,  so  that  one  sees  the  actual  land  forms 
in  relief,  although  one  gains  an  exaggerated  idea  of  the  relation  between 
the  vertical  and  the  horizontal.  Unfortunately,  the  expense  of  prepara- 
tion of  a  model  is  too  great  for  its  common  employment. 

In  some  instances,  elevations  are  shown  by  means  of  shading,  this 
being  known  as  the  hachure  method.     By  a  series  of  lines,  the  actual 


Fig.  266. 
Model  of  Cumberland  Valley,  Pennsylvania. 

elevations  are  made  to  appear  to  rise  above  the  rest  of  the  country, 
while  the  depressions  are  shown  in  their  natural  relation  to  the  high 
land.  This  method  is  used  by  the  United  States  Coast  Survey  in 
charting  the  coast  line  of  the  United  States  (Fig.  267),  and  it  is  employed 
in  some  of  the  European  countries.  Its  effect  is  very  vivid ;  but  one 
disadvantage  is,  that  while  the  differences  are  shown,  one  does  not  find 
information  concerning  the  actual  elevations  expressed  in  feet. 

The  contour  method  is  extensively  used,  and  is  employed  in  the  large 
scale  map  which  is  now  being  prepared  of  this  country.  While  from  the 
artistic  standpoint  it  is  not  so  effective  as  the  hachure  method,  it  is 

437 


438  PHYSICAL    GEOGRAPHY. 

superior  to  this  in  many  respects.  A  contour  is  a  line  of  equal  elevation. 
It  is  the  line  to  which  the  sea  would  rise  if  the  land  were  depressed  to 
the  depth  represented  by  the  height  of  the  line.  If  we  imagine  our- 
selves near  the  seashore,  the  coast  line  is  then  the  contour  line  of  0, 
and  the  100-foot  contour  line  is  that  to  which  the  sea  would  reach  if  it 
were  raised  just  100  feet. 

The  contour  map  (Figs.  150,  190,  228,  and  Plate  25)  is  made  upon 
a  horizontal  scale  which  varies  in  different  cases.  In  this  country  the 
usual  scale  is  one  inch  to  the  mile :  that  is,  every  mile  of  country  is 
allowed  one  inch.  No  allowance  is  made  for  the  vertical  element  of 
the  country.  Thus  if  a  region  of  considerable  irregularity  is  being 
mapped,  an  inch  on  the  sheet  is  made  to  represent  one  mile  in  a  hori- 
zontal dii'ection.  As  one  stands  upon  the  side  of  a  hill,  and  looks  across 
a  valley  to  another  hillside   at  the  same  elevation,  and  a  mile  distant, 


Fig.  267. 

the  horizontal  line  is  just  one  mile  in  length  ;  but  if  the  observer  should 
start  to  walk  from  the  place  where  he  stood,  to  the  point  to  which  he 
looked,  he  would  need  to  travel  considerably  more  than  a  mile.  On 
ordinary  maps  this  greater  distance  is  not  shown ;  but  on  the  contour 
maps  it  is  brought  out  by  means  of  the  contour  lines.  The  inch  repre- 
sents the  horizontal  mile.  Each  descent  or  ascent  finds  a  representation 
in  the  contour  lines ;  and  if  they  are  close  together,  one  sees  that  the 
vertical  distance  to  be  traveled  is  very  great. 

There  is  much  difference  in  the  scale  of  elevation  represented  by  con- 
tour lines.  On  most  of  the  maps  in  the  eastern  part  of  the  United  States, 
every  20  feet  of  ascent  or  descent  is  represented  by  a  contour  line,  and 
we  speak  of  this  as  the  contour  interval.  Let  us  suppose  ourselves  pass- 
ing over  an  irregular  country.     Imagine  that  we  are  to   travel  a  dis- 


APPENDIX  II.  439 

tance  of  one  mile,  in  the  course  of  which  we  go  down  into  one  valley,  up 
the  hillside  and  down  into  another  valley.  The  entire  area  on  the  map 
would  be  represented  in  the  space  of  one  inch.  If  the  first  valley  had 
a  depth  of  200  feet,  and  the  contour  interval  were  20  feet,  on  the  map 
representing  this  area  there  would  be  10  contour  lines,  which  would 
need  be  very  close  together,  because  the  descent  of  200  feet  in  the  small 
fraction  of  a  mile  would  necessarily  be  rather  rapid.  If  the  hill  over 
which  we  pass  rises  40  feet  above  the  valley  bottom,  we  would  ascend 
over  a  distance  represented  on  the  map  by  two  contour  lines,  —  a  rather 
moderate  ascent.  If  the  valley  on  the  opposite  side  of  the  hill  should 
happen  to  be  400  or  500  feet  in  depth,  the  descent  would  be  ex- 
tremely precipitous ;  and  it  would  be  necessary  to  represent  this  steep 
declivity  by  so  many  contour  lines  that  one  would  merge  into  the  other, 
and  there  would  be  a  mass  of  crowded  lines. 

From  the  several  sections  of  contour  maps  (Figs.  150,  190,  228,  and 
Plate  25,  reproduced  diagrammatically),  one  is  able  to  understand  the 
meaning  of  the  contour  lines,  and  to  discover  the  irregularities  which 
they  represent.^ 


REFERENCES. 

Nearly  every  European  government  is  publishing  a  topographic  map,  and 
among  these  are  to  be  found  many  excellent  illustrations  of  land  forms.  In 
this  country,  the  entire  area  of  Massachusetts,  Rhode  Island,  New  Jersey, 
and  Connecticut  is  now  mapped,  and  teachers  can  obtain  these  from  the 
Commissioners  of  the  Topographic  Map  at  the  state  capital.  In  all  of  the 
other  states  there  are  maps  of  some  districts ;  and  copies  of  these  may  be 
obtained  from  the  U.  S.  Geological  Survey.  During  the  year  1895-96  the 
Survey  will  issue,  at  a  small  price,  a  few  of  their  most  instructive  maps 
with  descriptive  text. 

The  seacoast  maps  of  the  U.  S.  Coast  Survey  are  excellent  and  cheap. 
The  same  is  true  of  the  maps  of  the  Great  Lakes,  the  Mississippi,  and  the 
Missouri.  A  very  important  pamphlet  ("  The  Use  of  Governmental  Maps  in 
Schools,"  Davis,  King  and  Collie,  Holt  &  Co.,  New  York,  1894,  .^0.30)  has 
been  prepared  for  the  purpose  of  indicating  useful  topographic  maps.  The 
methods  used  in  making  the  maps  of  the  Geological  Survey  are  described  in 
Gannett's  Manual  of  Topographic  Methods,  Monograph  XXII.,  U.  S.  Geo- 
logical Survey,  Washington,  1893.     4to.     $1.00. 

1  Specimen  maps  may  be  obtained  from  the  U.  S.  Geological  Survey. 


SUGGESTIONS   TO   TEACHERS. 

In  the  preparation  of  this  book,  the  endeavor  has  been  to  state  the 
subject  in  a  purely  descriptive  manner.  Nevertheless,  the  best  way  to 
learn  physical  geography  is  not  to  read  about  it,  but,  so  far  as  is  pos- 
sible, to  work  out  the  points  for  one's  self.  Not  merely  does  the  labo- 
ratory method  teach  the  subject  better,  but  it  trains  the  mind  of  the 
student  in  a  far  more  valuable  way  than  is  done  merely  by  acquiring 
information  from  a  book.  The  following  notes  are  appended  merely  as 
suggestions  concerning  the  way  in  which  simple  laboratory  methods  may 
be  introduced.  There  is  very  little  necessary  expense  attached  to  the 
introduction  of  these  methods  ;  but  of  course  by  the  acquirement  of  other 
and  more  expensive  materials  one  can  improve  the  teaching  almost  with- 
out limit. 

Each  teacher  will  need  to  work  out  the  details  of  the  problem  for  him- 
self;  for  the  environment,  the  available  materials,  the  time  that  can  be 
devoted  to  the  subject,  etc.,  are  so  variable  that  at  present  it  would  be 
difficult  to  outline  a  course  of  even  general  value.  I  would  urge  upon 
every  teacher  the  importance  of  introducing  some  laboratory  work ;  for 
it  will  stimulate  the  interest  of  the  student,  particularly  if  he  is  brought 
in  contact  with  the  real  phenomena  of  nature.  The  land  and  the  air 
are  always  available  and  full  of  lessons :  to  some,  the  ocean  or  the  lake 
shore  may  also  be  within  reach.  I  am  so  much  interested  in  having 
these  methods  introduced  that  I  invite  teachers  to  correspond  with  me, 
if  I  can  aid  them  in  obtaining  materials  for  teaching  purposes. 

Chapter  I.  —  Laboratory  work  in  illustration  of  this  chapter  is  not 
easy.  Still,  the  best  way  which  I  know  to  give  the  student  a  clear  idea  of 
the  relation  of  the  several  members  of  the  solar  system,  is  to  have  each 
student  construct  a  rough  model  of  it.  This  can  readily  be  done  by  means 
of  fine  wire  and  pasteboard.  By  merely  coiling  the  wire  on  the  desk,  each 
of  the  orbits  can  be  made  in  its  proper  relation  to  the  others.  Then  each 
planet  can  be  made  from  pasteboard,  the  size  representing  a  slice  cut 
along  the  equatorial  diameter.     In  order  to  have  this  produce  the  most 

440 


SUGGESTIONS   TO   TEACHERS,  441 

good,  the  scale,  or  relative  sizes  and  distances,  should  be  true  to  nature. 
Upon  these  orbits,  the  bodies  can  be  made  to  revolve  and  to  rotate,  so 
that  some  idea  may  be  obtained  concerning  the  relative  movements  of 
the  bodies.  The  relation  of  the  moon  and  earth  may  be  studied  in  the 
same  way. 

In  order  to  show  the  movements  of  the  earth  and  the  cause  of  seasons, 
an  excellent  method  is  to  construct  an  orbit  of  wire  and  cause  a  sphere 
to  move  around  it,  the  sphere  rotating  as  it  revolves.  There  are  various 
ways  in  which  this  may  be  done ;  a  permanent  orbit  may  be  constructed 
in  the  schoolroom,  and  a  large  ball,  or  better  a  globe,  may  be  carried 
around  it,  each  student  being  allowed  to  stand  near  the  center,  as  if  he 
were  in  the  position  of  the  sun.  Each  student  might  be  allowed  to  con- 
struct a  smaller  orbit  and  study  the  earth  movement  himself.  Celluloid 
spheres  are  very  inexpensive,  and  upon  them  the  continents  may  be 
roughly  outlined,  while  an  axis  is  passed  through  them  to  represent  the 
position  of  the  poles.  An  exercise  or  two  conducted  along  lines  some- 
thing like  the  above  will  do  more  to  teach  the  students  the  relations  of 
the  bodies  of  the  solar  system  than  a  score  of  lessons  from  the  book ; 
and  many  students  go  through  a  course  in  astronomy  without  a  proper 
conception  of  the  solar  system.  The  teacher  will  see  many  means  of 
adding  to  this  if  more  time  can  be  spared.  Thus  it  is  possible  to  show 
the  relations  of  the  comets  to  the  solar  system ;  the  immensity  of  the 
distance  to  the  stars  ;  the  size  of  the  sun ;  aphelion  and  perihelion ; 
apogee  and  perigee,  etc. 

Chapter  II.  —  The  teacher  of  physics  will  find  many  opportunities  for 
illustrating  this  chapter  by  laboratory  methods.  Thus  the  various  effects 
of  heat  and  light  are  capable  of  very  graphic  illustration.  Convection 
may  be  illustrated  by  heating  dust  or  smoke-filled  air  in  a  cylinder. 
Refraction  is  readily  shown  by  the  prism,  and  nearly  all  of  the  prin- 
ciples of  light  and  heat  may  be  illustrated.  Compression  of  air  can  be 
very  readily  shown.  Saturation  of  air  may  be  shown  by  placing  water 
in  the  bottom  of  a  cylinder ;  and  then  if  the  air  temperature  is  lowered, 
some  of  the  water  vapor  may  be  condensed  on  the  sides  of  the  vessel. 
The  various  ways  in  which  humidity  is  increased  or  decreased  can  be 
studied  in  detail  by  each  student ;  and  they  can  be  given  hypothetical 
cases  from  which  to  draw  conclusions  concerning  the  condition  of  the 
air  which  necessarily  follows. 

The  diiference  in  the  length  of  the  summer  and  winter  days  is  readily 
illustrated  by  the  use  of  the  globe  and  a  candle.     By  placing  the  candle 


442  PHYSICAL   GEOGRAPHY. 

in  different  positions,  so  as  to  throw  the  rays  at  the  angles  at  which  the 
solar  rays  reach  the  earth,  and  by  causing  the  globe  to  revolve,  this  is 
easily  seen  by  the  students. 

Chapter  III. — In  illustration  of  this  chapter,  laboratory  work  maybe 
introduced  by  stating  the  latitude  of  a  place  and  having  the  students  tell 
the  probable  temperature  conditions.  Then  add  the  altitude  and  have 
them  state  what  modifying  effect  this  would  have.  After  this  the  position 
with  reference  to  the  sea  may  be  given,  and  each  student  ought  to  be 
able  to  state  the  approximate  conditions  of  temperature.  They  could  be 
given  prominent  cities  in  the  world,  and  have  for  their  problem  the  deter- 
mination of  the  temperature,  for  which  purpose  it  would  be  necessary 
for  the  student  to  first  ascertain  their  position,  altitude,  etc.,  and  this 
would  also  serve  to  teach  geography.  On  the  other  hand,  given  a  set  of 
temperature  peculiarities,  the  students  can  determine  what  parts  of  the 
world  experience  them,  and  why  this  is  so.  The  teacher  can  tell  the 
student  of  differences  between  places  on  the  same  latitude,  or  of  resem- 
blances between  points  on  different  latitudes,  and  call  for  an  explanation 
of  these.  Much  similar  work  may  be  introduced  if  the  time  allows ;  and 
it  is  safe  to  say  that  not  only  will  the  interest  be  aroused,  but  the  habit 
of  logical  thought  will  be  improved. 

Each  student  can  construct  a  daily  curve  from  personal  observation, 
particularly  if  a  maximum  and  minimum  thermometer  are  available. 
With  either  fictitious  or  actual  data,  they  may  construct  a  seasonal  curve. 
Placing  a  maximum  and  minimum  thermometer  in  the  ground  at  a 
depth  of  one  or  two  feet,  the  difference  between  the  range  of  air  and 
earth  temperatures  is  very  vividly  impressed  upon  the  mind.  In  order 
to  make  this  even  more  striking,  temperature  observations  should  be 
kept  at  the  surface  of  the  ground,  and  at  an  elevation  of  about  10  feet. 
These  differences  are  best  shown  in  warm  weather. 

A  study  of  the  isothermal  charts  furnishes  opportunity  for  observation 
and  deduction,  particularly  if  Buchan's  charts  (see  p.  84)  can  be  ob- 
tained. The  student  can  construct  an  isothermal  chart  from  data  given 
and  averaged  for  several  places,  either  for  the  state,  or  the  country,  or 
for  the  locality  near  the  school.  For  these  and  other  purposes  in  which 
maps  are  needed,  the  set  of  cheap  outline  maps  published  by  Heath 
&  Co.  of  Boston,  or  Rand,  McNally  &  Co.  of  New  York,  are  valuable. 
Maps  of  all  the  states  and  territories  can  be  obtained.  The  data  of 
temperature,  etc.,  for  these  purposes  may  be  made  arbitrarily;  but 
it  would  be  better  to  use  the  tables  which  can  be  found  in  the  Annual 


fr 


SUGGESTIONS   TO   TEACHERS.  443 

Reports  of  the  Weather  Bureau.  In  some  states,  as  for  instance  in  New 
York,  climatic  data  will  be  found  in  the  State  Weather  Reports.  These 
and  the  national  reports  may  probably  be  obtained  free  of  cost,  provided 
a  statement  is  furnished  of  the  object  for  which  they  are  needed.  One 
report  w^ill  last  for  many  years.  With  these  data,  temperature  ranges 
and  other  illustrations  may  be  graphically  plotted  by  the  students.  The 
amount  of  laboratory  work  possible  in  this  and  other  subjects  far  exceeds 
the  time  that  will  be  available  in  most  schools. 

Chapter  IV.  —  After  studying  the  general  features  of  the  atmospheric 
circulation,  the  students  should  be  able  to  construct  a  summer  and  winter 
wind  chart  for  the  Pacific,  —  of  course  attempting  only  the  general  feat- 
ures. Upon  the  charts  of  the  Atlantic,  there  are  many  problems  which 
have  not  been  mentioned  in  the  text ;  and  a  thorough  examination  of 
the  wind  charts  wiU  be  valuable.  The  Challenger  charts  by  Buchan 
(see  p.  84)  contain  much  of  value  on  the  winds  of  the  globe.  As  an 
instance  of  how  observation  and  deduction  may  be  brought  into  the 
study,  the  follow^ing  might  be  suggested  as  a  fair  question  :  What  condi- 
tions result  in  the  two  opposite  seasons  in  the  belt  where  the  doldrums 
and  the  trade  winds  overlap? 

The  student  should  note  the  relation  between  wind  and  barometric 
conditions.  The  daily  weather  charts  ^  are  valuable  for  this  study ; 
and  the  student  can  also  make  his  own  observations  with  barometer, 
thermometer,  wind  vane,  etc.  A  particularly  valuable  study  can  be 
made  with  the  weather  maps.  By  examining  a  series  of  such  maps,  one 
may  observe  the  force  and  direction  of  the  winds,  and  the  progression 
of  the  conditions  favoring  certain  winds  during  the  successive  days.^ 

When  studied  with  reference  to  the  conditions  prevailing  in  its  home 
region,  this  method  becomes  of  much  value.  In  this  way  the  student 
can  come  into  the  possession  of  a  knowledge  of  the  causes  for  the  winds 
that  are  common  in  his  section,  as  well  as  the  relation  of  these  to  the 
winds  of  the  surrounding  country.  Observations  on  approximate  wind 
force  and  direction  can  easily  be  made  by  each  student ;  and  this  will  serve 
as  a  basis  for  a  comparative  study  of  the  daily  weather  maps.  Before 
the  map  of  the  day  is  shown  them,  they  should  be  able  to  approximately 
foretell  the  probable  conditions,  on  the  basis  of  a  series  of  simple  observa- 

1  The  teacher  can  probably  have  these  sent  by  mail  to  the  school. 

2  The  semi-daily  maps  are  of  especial  value  for  this  purpose,  and  some  of 
them  may  undoubtedly  be  obtained  by  applying  to  the  Weather  Bureau. 


444  PHYSICAL   GEOGRAPHY. 

tions  on  the  wind,  temperature,  and  pressure.  Such  a  study  will  create 
a  real  live  interest,  and  make  the  students  observers  of  the  things  of 
every-day  occurrence,  as  well  as  train  their  minds  to  the  habit  of 
drawing  logical  conclusions  from  a  series  of  observed  facts. 

Chapter  Y.  —  The  study  of  cyclones  and  anticyclones  receives  much 
aid  from  the  daily  weather  maps.  On  these  the  student  will  see  the  form 
and  size  of  the  areas,  their  rate  and  direction  of  progression,  the  amount 
and  distribution  of  rainfall,  the  direction  of  the  winds,  their  spiral 
tendency,  the  left-hand  whirling,  etc.  He  will  observe  how  the  winds 
change  from  day  to  day,  and  what  relation  they  bear  to  the  areas  of 
high  and  low  pressure.  He  can  predict  the  changes  and  study  them 
in  connection  with  the  weather  of  his  own  immediate  neighborhood. 
The  storm  paths  and  their  irregularities  can  be  studied  with  the  aid  of 
the  Monthly  Weather  Reviews.^  From  the  weather  predictions,  and 
the  printed  notes  on  the  map,  the  relation  between  the  cyclonic  areas  and 
thunderstorms  is  readily  seen.  The  Coast  Pilot  ^  for  the  fall  months, 
often  contains  valuable  material  for  study  in  connection  with  West  Indian 
hurricanes. 

Chapter  VI.  —  The  student  can  be  directed  in  the  study  of  the  forma- 
tion and  movement  of  clouds,  and  their  relation  to  rainfall  and  tempera- 
ture. Reports  upon  these  observations,  from  time  to  time,  will  stimulate 
them  to  a  deeper  interest  in  cloud  formation.  Attention  can  be  directed 
to  the  possibility  of  predicting  weather  changes  by  an  examination  of  the 
clouds.  This  furnishes  an  excellent  opportunity  for  bringing  the  student 
into  contact  with  nature.  A  study  of  the  rainfall  charts,^  in  connection 
with  those  of  temperature  and  wind,  will  give  opportunity  for  the  ex- 
planation of  many  peculiarities  of  rainfall  distribution.  Careful  obser- 
vation concerning  the  rainfall  of  the  place  where  the  stadent  lives  will 
be  of  value  in  showing  the  irregularities  in  amount,  as  well  as  in  occur- 
rence.    Let  him  compare  this  with  that  of  the  doldrum  belt. 

A  sling  psychrometer  (see  Appendix  I.)  may  be  readily  constructed  from 
two  thermometers,  and  the  relative  humidity  of  the  air  be  determined. 
From  this  the  student  can  be  taught  to  predict  the  occurrence  of  dew 
or  frost  for  the  succeeding  nights.  The  value  of  these  lessons  will  be 
greatly  increased  if  the  students  are  callea  upon  for  reports.     If  the  pre- 

1  These  also  may  probably  be  obtained  at  Washington  upon  application. 

2  Distributed  free  by  the  Hydrographic  Bureau  of  the  Navy  Department. 
8  Those  recently  published  by  the  Weather  Bureau  are  very  valuable. 


SUGGESTIONS   TO   TEACHEBS.  445 

dictions  that  are  made  are  not  fulfilled,  perhaps  the  reasons  will  be 
apparent;  and  there  may  have  been  dew  at  one  place  and  frost  at 
another,  or  dew  at  one  home  and  none  at  another.  Then  the  explana- 
tions for  these  differences  can  be  obtained  from  the  students. 

There  are  few  better  ways  to  train  the  habit  of  observation  than  to 
tell  the  students  to  look  for  certain  things,  giving  enough  directions  so 
that  they  may  be  led  to  observe.  K  too  little  guidance  is  given,  all  but 
the  brightest  will  be  appalled  by  the  difficulties;  for  one  of  the  least 
developed  parts  of  the  student  mind  is  generally  that  which  directs  the 
eye  to  look  for  details,  and  then  to  put  these  details  together  into  a  con- 
nected whole.  I  have  often  noticed  how  pleased  secondary  school  stu- 
dents have  been  when  their  teacher  has  told  them  to  look  up  something, 
and  with  what  earnestness  they  have  worked  to  have  correct  answers. 
They  like  to  be  made  to  feel  that  they  are  using  their  own  minds; 
and  it  is  a  distinct  relief  from  the  monotony  of  learning  what  the  book 
says.  Since  they  are  in  constant  contact  with  the  problems  of  physical 
geography,  each  day  can  be  made  to  yield  opportunity  for  observation ; 
and  nothing  could  be  more  profitable  than  to  give  the  class  a  daily  task 
in  observation,  devoting  a  part  of  the  recitation  hour  to  a  discussion  of 
the  results. 

Chapter  VII.  —  Many  of  the  suggestions  made  for  the  previous  chap- 
ters will  apply  to  this ;  but  there  are  many  ways  in  which  these  may  be 
put  together  for  a  whole.  The  probable  conditions  of  weather  and  climate 
in  various  parts  of  the  earth  may  be  inferred  by  a  study  of  the  charts  of 
temperature,  wind,  and  rain.  The  country  near  the  school  may  furnish 
illustration  of  local  differences  in  climate. 

Chapter  VIII.  —  There  is  of  course  much  opportunity  for  an  enlarge- 
ment of  this  subject ;  but  it  would  come  better  under  a  study  of  zoology 
and  botany.  The  object  sought  in  this  chapter  is  to  point  out  the  rela- 
tion between  climate  and  life,  and  to  show  also  that  the  land  itself 
presents  certain  obstacles  to  the  spread  of  life. 

Chapters  IX.,  X.,  and  XL  —  Unless  the  student  dwells  by  the  sea- 
shore, there  is  little  of  value  to  be  obtained  from  an  attempt  at  observa- 
tion study  in  the  topics  covered  by  these  chapters.  The  charts  may  be 
studied,  and  reasons  found  for  the  peculiarities  exhibited.  If  the  charts 
of  the  Challenger  Keports  are  available,  particularly  those  accompany- 
ing the  two  final  volumes  of  Summary,  there  will  be  found  much 
opportunity  for  laboratory  study.  By  a  careful  examination  of  the 
charts  of  the  ocean  bottom,  much  can  be  learned  concerning  the  topog- 


446  PHYSICAL   GEOGBAPHY. 

raphy  of  that  large  part  of  the  earth's  surface  which  is  submerged 
beneath  the  ocean.i 

One  or  two  visits  to  the  seashore  for  the  purpose  of  studying  the 
rise  and  fall  of  the  title,  the  action  of  waves,  the  distribution  of  life 
along  the  coast,  etc.,  will  be  of  great  value.  Even  upon  the  shore  of  a 
lake  some  of  these  features  may  also  be  illustrated.  The  distribution  of 
cold  and  warm  water  by  the  ocean  currents,  furnishes  much  opportunity 
for  study  in  connection  with  climate. 

For  tides,  the  "Tide  Tables  "  (see  reference  at  end  of  Chapter  XL) 
give  data  for  a  very  interesting  study.  The  rise  and  fall  of  the  tide  is 
stated  for  various  places;  and  if  the  student  is  told  to  construct  a  dia- 
gram similar  to  Fig.  86,  which  is  based  upon  these  tables,  he  wHll  learn 
much  about  the  rise  and  fall  of  the  tide.  At  the  end  of  that  book  the 
phases  of  the  moon  and  the  times  of  perigee  and  apogee  are  stated,  so 
that  the  reasons  for  many  of  the  more  important  tidal  variations  will 
become  apparent  after  a  little  study  and  thought.  Taking  the  various 
stations  for  which  the  tidal  predictions  are  tabulated,  and  locating  them 
upon  a  map,  one  sees  the  geographic  reasons  for  the  difference  in  tidal 
height  from  place  to  place  ;  and  given  a  place  with  a  certain  geographic 
location,  the  student  can  apply  these  principles  to  the  approximate  deter- 
mination of  the  tidal  conditions.  There  is  no  better  way  to  impress 
upon  the  student  the  peculiarity  of  tidal  movement,  than  to  have  him 
laboriously  construct  a  chart  of  these  movements.  For  students  of  the 
interior,  this  is  less  important  than  for  those  who  dwell  near  the  sea. 

Chapter  XII.  —  There  is  almost  no  limit  to  the  opportunity  for  field 
and  laboratory  study  upon  the  topics  briefly  outlined  in  this  chapter, 
though  it  more  properly  falls  to  the  province  of  geology.'^  Photographs 
of  various  phenomena,  as  well  as  lantern  slides  made  from  them,  are  now 
easily  obtained.^  An  excellent  method  is  to  project  a  view  upon  the 
screen,  and  call  upon  students  for  a  description  of  the  phenomena  illus- 
trated. This  has  the  great  advantage  of  placing  an  enlarged  picture 
before  the  class,  so  that  each  student  may  see  every  feature ;  and  this 
does  away  with  the  necessity  of  many  duplicate  pictures  with  one  in  the 
hand  of  each  student.     Where  the   latter   method   is   employed,  cheap 

1  The  Jones  relief  globe,  sold  by  A.  H.  Andrews  &  Co.,  215  Wabash 
Ave.,  Chicago,  111.,  for  $100,  is  of  great  value  in  this  connection. 

2  The  study  of  geology  could  very  properly  be  introduced  here  as  a  part  of 
physical  geography. 

^  The  author  will  be  glad  to  advise  teachers  who  wish  to  obtain  these. 


SUGGESTIONS   TO   TEACHERS.  447 

blueprints  may  serve  admirably  as  substitutes.  With  the  widespread 
introduction  of  electricity,  it  is  now  possible,  in  many  schools,  to  make 
use  of  the  electric  lantern,  which  may  be  used  in  a  room  only  partially 
darkened.  Much  can  be  done  by  asking  the  students  to  describe  the 
features  illustrated  in  the  pictures  in  the  book.  Several  phenomena  are 
often  illustrated  in  the  same  view. 

By  far  the  best  way  to  study  the  phenomena  of  the  earth's  surface  is 
to  see  the  actual  thing;  and  there  are  usually  opportunities  for  some 
such  study  near  the  school.  In  most  cases  the  teacher  can  find  some 
phenomena  of  geology,  such  as  igneous  or  sedimentary  rocks,  fossils, 
folds,  etc.     The  students  will  enjoy  and  profit  by  field  excursions. 

Collections  of  the  common  minerals  and  rocks  can  be  bought  for  a 
few  dollars ;  ^  and  more  will  be  learned  by  an  hour's  study  of  such  a  col- 
lection, than  by  weeks  of  study  from  the  books.  Some  of  the  common 
rocks  and  minerals  may  usually  be  collected  near  the  school.  The 
teachers  in  those  schools  which  are  located  within  the  glacial  belt  will 
find  a  storehouse  of  rock  specimens  in  the  clay  and  gravel  banks.  All  of 
the  common  rocks,  and  many  of  the  minerals,  will  often  be  found  there. 
If  the  students  can  be  sent  or  taken  out  for  the  purpose  of  making  such 
collections,  they  will  soon  learn  a  great  deal  about  rocks  ;  and  this  plan 
will  be  found  admirable,  even  if  the  school  has  complete  collections. 

Chapter  XIII.  —  In  most  places  the  phenomena  of  erosion  and 
weathering  can  be  studied  in  the  field.  Rock  specimens  exposed  at 
the  surface  will  show  the  destruction  in  progress;  and  upon  exposed 
bluifs  many  instructive  lessons  may  be  studied.  A  journey  in  such  a 
place  will  be  found  to  be  most  profitable;  and  the  students  will  see 
important  things  that  the  majority  of  the  world  pass  by  without  ever 
noticing.  A  visit  to  a  spring  may  prove  of  value ;  and  if  it  chances 
to  contain  iron,  or  other  substances,  in  solution,  chemical  action  of  water 
becomes  something  more  than  the  mere  book  statement. 

In  most  parts  of  the  country,  wind  and  glacial  action  cannot  be  illus- 
trated by  actual  examples.  Upon  the  lake  shore,  or  better  upon  the  sea- 
shore, wave  action  may  be  studied ;  and  in  practically  every  part  of  the 
country,  some  form  of  river  and  rain  erosion  may  be  seen.  Let  the 
teacher  have  the  students  watch  the  rills  and  brooks  and  report  upon 
the  change  in  amount  of  water  and  sediment.     This  will  train  their 

1  Ward's  Natural  Science  Establishment  at  Rochester,  KY.,  and  E.  E. 
Howell,  612  17th  St.,  N.W.,  Washington,  D.C.,  have  such  collections. 


448  PHYSICAL   GEOGBAPHY. 

powers  of  observation  and  arouse  their  interest;  and  the  skillful  teacher 
may  make  this  the  basis  upon  which  to  build  a  real  understanding  of 
the  action  of  rivers.  The  key  to  success  in  this  direction  is  to  tell  the 
student  only  so  much  as  is  absolutely  necessary,  but  to  make  him  tell  the 
story,  not  from  memory  of  what  the  book  says,  but  upon  the  basis  of  a 
series  of  observations  which  necessarily  lead  to  these  conclusions. 

It  is  not  necessary  to  find  illustrations  of  all  phenomena  in  the  field, 
though  the  more,  the  better ;  but  the  object  is  to  teach  the  student  how  to 
see  for  himself,  so  that  he  may  see  other  illustrations  whenever  he  hap- 
pens to  come  upon  them.  Where  it  is  not  feasible  to  study  the  phe- 
nomena in  the  field,  photographs  or  lantern  slides  make  a  fair  substitute. 

Chapter  XIV.  —  With  a  set  of  Physical  Maps^  of  the  continents, 
there  is  opportunity  for  study  of  the  grander  features  of  the  land.  These 
are  much  more  naturally  shown  upon  a  relief  globe.^  The  distribution 
of  mountains,  continents,  seas,  etc.,  are  there  shown  very  vividly.  Par- 
ticularly is  this  the  case  in  the  second  model,  for  here  the  ocean  waters 
are  not  present  to  obscure  the  topography  of  the  bottom  of  the  sea. 

The  larger  features  of  the  United  States  may  be  studied  on  the 
nine-sheet  contour  map  published  by  the  U.  S.  Geological  Survey; 
and  also  upon  the  smaller  shaded  relief  map  published  by  the  same 
bureau.^  Better  still,  if  the  school  can  afford  it,  a  model  of  the  United 
States  should  be  obtained.*  With  these  aids,  a  good  knowledge  of  the 
geography  of  the  world  can  be  obtained,  and  at  the  same  time  much 
training  be  gained,  for  the  teacher  will  find  ample  opportunity  to  suggest 
problems  for  the  pupil  to  study  and  answer. 

Chapter  XV.  —  In  many  parts  of  the  country,  particularly  within 
the  glacial  belt,  two  types  of  river  valley  may  be  seen  within  a  short 
distance  of  the  school ;  and  everywhere  in  the  field  it  will  be  possible 
to  see  illustration  of  some  stage  of  river-valley  development.  The  teacher 
can  make  such  an  excursion,  or  series  of  excursions,  the  basis  for  an  ex- 
pansion of  the  subject  of  river-valley  development.   Where  these  features 

1  The  Kiepert  maps  are  sold  by  most  large  dealers  in  school  supplies. 
I?and,  McNally  &  Co.  of  Chicago  and  New  York  advertise  a  similar  set. 

2  Such  as  that  sold  by  Rand,  McNally  &  Co.  of  New  York,  or  the  Jones 
globe  (see  suggestions  for  Chapter  XI.). 

3  The  latter  accompanies  the  Thirteenth  Annual  Report  of  the  Survey. 

4  E.  E.  Howell,  612  17th  St.,  N.W.,  Washington,  has  a  model  of  the 
country  for  $125,  and  a  smaller  one  for  $25. 


SUGGESTIONS   TO   TEACHEBS.  449 

are  not  well  illustrated,  recourse  may  be  had  to  photographs  or  lantern 

slides. 

A  study  of  topographic  maps  will  be  found  of  great  value  in  this 
connection,  as  well  as  in  illustration  of  the  features  described  in  the 
following  chapters.  For  suggestions  concerning  the  special  maps  needed, 
and  their  use,  see  the  pamphlet  by  Davis,  King,  and  Collie,  referred  to 
at  the  end  of  Appendix  II. 

Chapter  XVI.  —  Here  again  there  is  the  possibility  of  finding  illus- 
trations in  the  field,  and  a  certainty  of  finding  them  in  photographs  and 
slides.  The  U.  S.  Geological  Survey  topographic  map  of  Niagara  (free), 
and  the  Lake  Survey  map  of  the  same  (United  States  Engineer  Office, 
34  W.  Congress  St.,  Detroit,  Michigan ;  ^0.20),  are  very  useful.  The 
latter  bureau  publishes  a  number  of  charts  of  the  Great  Lakes ;  and  on 
the  Geological  Survey  maps,  notably  those  of  New  England,  many 
illustrations  of  glacial  lakes  and  swamps  will  be  found.  The  Mississippi 
delta  is  well  illustrated  on  the  U.  S.  Coast  Survey  chart  194.  For  flood- 
plain  peculiarities,  see  particularly  the  maps  published  by  the  Mississippi 
and  Missouri  River  Commission,  whose  headquarters  are  at  St.  Louis, 
Missouri.  For  facts  concerning  these  maps  see  the  pamphlet  by  Davis, 
King,  and  Collie,  referred  to  at  the  end  of  Appendix  II. 

Chapter  XVII.  —  The  effects  of  glaciers  upon  the  surface  of  the 
land  may  be  partly  inferred  from  the  study  of  a  series  of  topographic 
maps  of  places  within  the  glacial  belt,  and  a  comparison  of  these  with 
some  from  outside  of  this  belt.  This  may  be  very  well  supplemented 
by  views  from  the  two  regions ;  and  then,  if  the  school  is  situated 
within  the  glacial  belt,  by  excursions  ^  to  glacial  deposits.  These  will 
be  of  great  value  for  the  illustrations  of  many  points.  In  all  of  these 
cases,  the  teacher  should  have  the  students  observe  as  much  as  possible, 
and  should  avoid  telling  them  things  which  they  ought  to  be  able  to  see 
for  themselves. 

Chapter  XVIII.  —  A  teacher  who  has  given  no  attention  to  the  sub- 
ject, will  be  astonished  to  find  how  many  lessons  can  be  learned  by  an 
hour's  tramp  on  the  shore  of  a  lake  or  the  ocean.  The  beaches  and  cliffs 
are  full  of  interest;  and  on  some  ocean  coasts,  as  well  as  on  most  lake 
shores,  there  will  be  found  numerous  instances  of  the  minor  coastal  feat- 

1  The  number  of  excursions  suggested  may  seem  excessive  ;  but  it  is 
assumed  that  no  one  school  will  be  so  favorably  located  as  to  make  it  possible 
to  study  all  of  these  phenomena  in  the  field. 

2g 


450  PHYSICAL   GEOGRAPHY, 

ures,  such  as  bars,  spits,  and  possibly  small  deltas.  This  kind  of  work 
may  very  advantageously  be  supplemented,  or  if  necessary  be  replaced,  by 
a  study  of  the  admirable  charts  of  the  American  coast,  which  are  sold 
at  a  very  slight  cost  by  the  U.  S.  Coast  Survey  at  >Vashington.  Some 
of  these  charts  should  be  in  every  school  where  physical  geography 
is  taught.  For  those  who  dwell  near  the  Great  Lakes,  the  charts  of 
the  Lake  Survey  (sold  at  $0.20  a  sheet)  will  be  found  very  valuable  aids 
to  the  study  of  shore  lines. 

Chapters  XIX.  and  XX.  —  To  most  students  the  subjects  treated  in 
these  chapters  are  inaccessible,  and  they  nmst  be  studied  upon  maps, 
models,  and  photographs.  Unfortunately  the  demand  for  materials  for 
laboratory  instruction  in  geology  and  physical  geography,  has  not  yet  been 
sufficient  to  warrant  the  preparation  of  cheap  illustrative  models  of  such 
phenomena  as  these.  In  schools  where  modeling  is  done,  many  valuable 
lessons  could  be  taught  by  having  each  student  illustrate  these  changes 
by  the  actual  construction  of  models ;  and  a  well-constructed  series 
would  undoubtedly  find  ready  sale.  As  soon  as  the  rational  method 
of  instruction  is  introduced,  and  there  is  a  strong  demand  for 
new  and  additional  material,  it  will  undoubtedly  be  supplied.  In  the 
meantime  it  will  be  necessary  for  the  teacher  to  make  use  of  the 
only  material  that  is  at  hand ;  namely,  maps  and  photographs.  Much 
can  be  learned  from  a  carefully  selected  series  of  these ;  and  some  of  the 
schools  will  be  situated  near  or  among  the  mountains,  so  that,  in  these 
cases,  excursions  may  be  made  for  the  purpose  of  studying  some  of  the 
mountain  peculiarities.  In  many  parts  of  New  England,  in  the  Catskills, 
and  in  the  entire  Appalachian  belt,  the  opportunity  for  this  kind  of 
illustration  is  excellent ;  and  if  the  teacher  will  take  the  trouble  to  look 
about  him,  he  will  find  numerous  interesting  lessons.  For  instance, 
along  the  eastern  base  of  the  Appalachians,  there  exist  two  sets  of  moun- 
tain ranges,  the  very  ancient  series  now  reduced  to  low,  rounded  hills, 
and  the  younger,  but  still  old,  and  relatively  high  Appalachians. 
Among  the  Cordilleras,  there  is  an  abundant  opportunity  for  the  study 
of  mountains,  and  in  many  places  of  volcanoes  also. 

Chapter  XXI.  —  The  teacher  will  be  able  to  illustrate  these  features 
also;  for  by  looking  about  him,  he  will  find  a  variety  of  land  forms,  and 
among  these  will  be  found  illustrations  of  importance.  They  may  be 
merely  plains  or  swamps,  or  they  may  be  mountains.  For  the  teacher 
who  looks  with  an  open  eye,  there  is  abundant  chance  for  the  discovery 
of  illustrations  of  the  relation  between  structure  and   topography.      It 


SUGGESTIONS   TO   TEACHERS.  451 

would  not  be  necessary  to  take  excursions  to  every  place ;  but  an  admi- 
rable method  is  to  request  the  students  to  visit  some  of  the  places  and 
report  upon  them.  This  method  has  been  tried  with  good  success,  the 
students  being  sent  out  in  squads  to  examine  and  report  upon  land  or 
rock  peculiarities,  at  times  outside  of  the  regular  school  hours.  There 
are  many  photographs  and  maps  which  may  be  used  in  illustration  of 
this  chapter. 

Chapter  XXII.  —  The  teacher  will  find  it  possible  to  expand  this 
subject  in  connection  with  the  study  of  history  and  geography.  Indeed, 
throughout  physical  geography  there  are  numerous  points  which  could 
properly  be  made  to  serve  in  the  teaching  of  these  subjects.  Much  good 
can  be  done  in  geographic  teaching  by  showing  that,  in  many  cases,  features 
of  geographic  importance  are  not  arbitrary,  but  have  their  origin  in  phys- 
ical causes.  Most  of  us  have  learned  that  England  is  a  great  country, 
that  it  manufactures  this  and  that,  etc. ;  but  the  fundamental  reasons  for 
her  greatness  are  not  ordinarily  presented.  We  learned  to  bound  Switzer- 
land or  France,  but  did  not  learn  what  these  boundaries  meant.  We 
learned  the  size,  position,  and  industry  of  Philadelphia,  but  did  not  find 
out  the  reasons.  If  we  had  been  told  the  causes,  the  isolated  fact  would 
have  been  more  easily  retained;  for  the  average  mind  learns  unconnected 
facts  with  much  less  ease  than  those  which  are  philosophically  related. 

Chapter  XXIII.  —  This  chapter  is  mainly  intended  to  be  one  of 
information;  and  while  abundant  opportunity  exists  for  laboratory 
work,  it  does  not  seem  so  essential,  or  so  easily  obtained,  as  in  the  pre- 
ceding chapters.  Indeed,  the  teacher  who  follows  the  foregoing  sug- 
gestions will  probably  find  that  the  main  difficulty  lies  in  the  fact  that 
too  much  is  suggested. 

Appendix  I.  —  No  part  of  meteorology  is  better  capable  of  furnishing 
illustration  by  laboratory  methods.  The  various  instruments  can  be 
placed  by  the  teacher,  and  the  class  be  taught  to  make  regular  observations, 
just  as  is  done  at  any  meteorological  station.  These  can  be  plotted  upon 
cross-section  paper,  to  illustrate  ranges  in  temperature,  weather  changes, 
etc.  By  this  means  each  of  the  more  important  instruments  may  be 
understood,  and  a  knowledge  obtained  concerning  the  results  of  their 
use.  A  series,  of  weather  maps  for  successive  days  can  be  furnished 
each  student  for  study,  and  for  statement  concerning  the  conditions  and 
changes  illustrated.  Much  interest  in  the  subject  will  be  aroused  by 
having  the  weather  map  posted  in  some  conspicuous  place;  and  each 
student  can  be  taught  to  see  upon  what  basis  the  weather  predictions 


452  PHYSICAL   GEOGRAPHY. 

are  made.  Indeed,  the  students  may  make  their  own  weather  maps 
and  weather  predictions.  Furnished  with  outline  maps  of  the  United 
States,^  each  student  can  plot  temperature,  pressure,  wind  direction, 
etc.,  for  various  places,  from  observations  which  the  teacher  furnishes 
from  a  map.  After  making  one  or  two  of  these,  the  student  will  be 
in  a  position  to  thoroughly  understand  weather  maps.  Let  the  teacher 
take  a  series  of  weather  maps  for  successive  days,  and  have  the  class 
plot  upon  their  maps  the  conditions  there  recorded.  After  two  or  three 
have  been  finished,  each  member  of  the  class  ought  to  be  able  to  make  a 
fairly  close  prediction  of  the  general  weather  conditions  of  the  country 
for  the  next  day.  They  might  even  embody  these  predictions  upon 
another  map.  Not  only  will  these  methods  teach  students  how  to  use 
weather  maps,  but  the  mind  is  put  to  work  imagining  and  drawing  con- 
clusions from  a  series  of  facts. 

Weather  maps  are  readily  obtained  free  of  cost  from  the  United  States 
Weather  Bureau,  at  Washington ;  and,  in  some  states,  a  teacher  who  is 
willing  to  maintain  voluntary  observations  may  obtain  the  more  common 
instruments  from  the  State  Weather  Bureau.  A  set  of  the  really  neces- 
sary instruments  is  not  so  very  expensive,  and  some,  such  as  the  ther- 
mometer and  barometer,  may  also  be  used  in  the  physical  laboratory. 

Appendix  II.  —  There  is  no  better  way  to  teach  the  student  the  mean- 
ing of  the  topographic  map,  than  to  have  him  make  one  of  a  small  area. 
Moreover,  it  impresses  the  meaning  of  elevations  in  a  way  that  no  other 
in-door  method  can  do.  In  the  making  of  models  and  maps,  there  is  a' 
training  in  the  appreciation  of  proportion,  in  constructive  imagination, 
and  in  the  grouping  of  facts,  that  is  most  valuable,  and  is  usually  not 
obtained  by  the  student.  No  one  should  be  allowed  to  go  through  the 
secondary  school  without  having  some  development  of  the  "  topographic 
sense."  I  have  known  educated  people  who  have  lived  in  a  place  for 
several  years  without  having  the  points  of  the  compass  in  mind,  who 
have  had  no  idea  of  the  direction  to  a  neighboring  place  to  which  they 
have  gone  by  train  or  wagon,  and  whose  estimate  of  distance  is  simply 
ridiculous.  Particularly  is  this  true  of  women :  for  most  men,  by  contact 
with  the  outer  world,  learn  by  experience  what  they  might  easily  have 
been  taught  in  school,  while  the  majority  of  women  get  little  of  this 
training,  even  by  experience. 

1  Such  as  those  sold  by  Rand,  McNally  &  Co. ,  of  Chicago,  and  Heath  & 
Co.,  of  Boston,  at  the  rate  of  a  few  dollars  a  thousand. 


QUESTIONS   UPON  THE   TEXT. 

In  the  following  questions,  no  attempt  is  made  to  include  all  that 
could  possibly  be  asked,  but  rather  to  ask  the  most  important,  and  indi- 
cate what  class  of  questions  seems  best  calculated  to  produce  the  most 
desirable  effect,  both  in  interesting  the  student,  and  in  drawing  from 
him  what  he  knows.  The  questions  frequently  ask  for  a  general  view 
of  the  subject ;  and  it  may  often  be  necessary  for  the  teacher  to  ask  the 
pupil  other  questions  which  shall  aid  in  obtaining  a  thorough  answer. 
An  excellent  kind  of  question,  is  one  calling  for  more  than  a  mere 
answer  from  the  text,  but  rather  one  in  which  the  student  groups 
things,  partly  from  his  own  mind,  and  partly  from  the  book ;  such,  for 
instance,  as  asking  the  application  or  bearing  of  a  point  treated  in  the 
book.  The  questions  are  arranged  under  sections  corresponding  to  those 
of  the  book,  and  usually  follow  the  order  of  presentation  of  the  subject. 

CHAPTER  I. 
The  Earth  as  a  Planet.    Pages  3-22. 

Form  of  the  Earth.  —  Of  what  is  the  earth  composed?  What  is  its 
form?  What  irregularities  are  there  on  the  surface?  What  are  the 
differences  between  the  elevation  of  the  land  and  the  depth  of  the  ocean? 
What  is  the  area  of  land  and  water  ?  What  is  the  depth  of  the  atmos- 
phere ? 

The  Solar  System.  — What  are  the  five  classes  of  members? 

The  Sun.  —  How  does  the  sun  differ  from  the  other  members  of  the 
solar  system?  What  does  the  spectroscope  reveal?  What  are  the  three 
parts  of  the  sun?  The  characteristics  of  each?  What  are  sun  spots? 
What  are  the  movements  of  the  sun  ? 

The  Planets.  —  What  are  the  important  features  of  Mercury?  Of 
Venus?   Of  Mars?    Of  Jupiter?   Of  Saturn?  Of  Uranus?    Of  Neptune? 

Asteroids.  —  What  are  these  ? 

453 


454  PHYSICAL   GEOGRAPHY. 

The  Earth.  —  What  reasons  have  we  for  believing  that  the  interior  is 
highly  heated  ?  What  is  the  probable  condition  of  the  interior  ?  What 
are  the  movements  of  the  earth?  What  are  the  peculiarities  of  its 
revolution  ?     What  is  the  cause  of  the  seasons  ? 

The  Moon.  —  What  are  its  movements ?  What  is  perigee?  Apogee? 
Why  is  one  side  of  the  moon  never  seen  from  the  earth  ?  What  are  the 
probable  conditions  on  the  moon  ? 

Comets,  Shooting  Stars,  and  Meteors.  —  What  are  comets?  How  do 
they  move?    What  is  the  origin  of  meteors ?    Why  do  they  glow? 

The  Stellar  System.  —  What  is  the  probable  number  and  distance  of 
the  stars  ?     How  are  they  arranged  ?     What  and  where  are  nebulae  ? 

Symmetry  of  Solar  System.  —  What  points  of  symmetry  are  noticed? 
What  are  the  distances  between  the  members  ?     Illustrate. 

The  Nebular  Hypothesis.  —  State  it. 

Verification  of  the  Nebular  Hypothesis.  —  What  points  are  there  tend- 
ing to  verify  this  hypothesis?    What  is  the  probability  of  its  truth? 

CHAPTER  n. 
The   Atmosphere.     Pages  23-42. 

General  Statement.  —  What  variation  is  there  in  the  density  of  the 
air?  What  gases  compose  the  atmosphere?  What  is  dust  in  the  atmos- 
phere ?     Water  vapor  ?     What  is  the  importance  of  the  atmosphere  ? 

Light.  —  What  is  the  source  of  our  light?  Of  what  is  white  light 
composed?  What  is  diffusion  of  light?  Selective  scattering?  What 
effect  upon  light  is  produced  by  dust?  What  is  the  cause  for  the  sunset 
color  ?  What  is  reflection  ?  Give  illustration.  What  is  mirage  ?  Loom- 
ing? Refraction?  What  is  the  cause  of  the  rainbow?  What  is  the 
halo?  The  corona?  What  is  absorption  of  light?  Why  are  bodies 
transparent,  translucent,  and  opaque?     Why  are  some  objects  colored? 

Electricity  and  Magnetism.  —  What  are  the  indications  of  terrestrial 
magnetism  ?  How  is  atmospheric  electricity  made  apparent  ?  What  is 
lightning?     Thunder?     Heat  lightning ? 

Heat.  —  What  is  the  source  of  heat  ?  How  do  different  bodies  behave 
toward  it?  What  interferes  with  its  passage  through  the  atmos- 
phere? Why  does  the  ocean  surface  remain  relatively  cool?  What  is 
latent  heat?  Why  does  the  land  become  warmer  than  the  ocean? 
How  is   the   atmosphere  warmed?     What  is    radiation?    Conduction? 


QUESTIONS    UPON    THE  TEXT.  455 

Convection?  What  is  the  importance  of  convection?  What  are  the 
differences  in  heat  effect  and  their  results  ?  What  is  the  effect  of  rota- 
tion on  the  temperature  of  the  air  ?  Of  revolution  ?  How  does  this 
differ  in  various  parts  of  the  earth?  What  are  the  reasons  for  the  short, 
cold  days  of  the  temperate  latitude  winter  ?  What  is  the  normal  varia- 
tion or  range  in  temperature  during  the  year  ?  How  does  this  differ  in 
the  several  zones,  —  tropical,  temperate,  and  arctic  ? 

Moisture.  —  What  is  evaporation?  What  is  saturated  air?  In  what 
places  is  the  air*  naturally  driest  ?  Why  do  winds  favor  evaporation  ? 
How  does  temperature  effect  evaporation?  What  is  absolute  humidity? 
Relative  humidity?  Dew-point?  What  is  the  effect  upon  humidity 
caused  by  oceans?  By  tropical  heat ?  By  elevation?  By  descent  of  air 
from  higher  altitudes?  By  the  passage  of  air  currents  from  warm  to 
cold  regions  ?  From  cold  to  warm  ?  By  the  rising  of  air  ?  What  are 
the  effects  of  variations  in  humidity? 

Pressure.  —  In  what  two  ways  does  the  air  pressure  vary? 

Effect  of  Gravity.  —  What  is  its  effect  upon  the  atmosphere  ? 

Effect  of  Rotation.  —  What  important  effect  upon  moving  bodies  of  air 
and  water  is  produced  by  the  earth's  rotation  ?     State  the  reason. 

CHAPTER  III. 
Distribution  of  Temperature.     Pages  43-67. 

General  Statement.  —  What  is  the  normal  distribution  of  temperature 
from  equator  to  pole  ?  What  are  the  normal  seasonal  and  daily  ranges 
or  curves ?     How  are  they  interfered  with? 

Effect  of  Atmospheric  Movements.  —  In  what  ways  do  the  atmospheric 
movements  modify  the  temperature  ? 

Influence  of  Oceans.  —  Why  are  the  ocean  temperatures  more  equable 
than  those  of  the  land?  What  is  the  effect  of  the  oceanic  circulation  in 
this  respect?  How  does  the  temperature  change  from  seashore  to  in- 
terior ?     From  tropical  to  arctic  regions  ? 

Effect  of  Topography.  —  How  does  the  temperature  on  the  hills  differ 
from  that  of  the  valleys  ?  How  does  it  differ  on  the  north  and  south 
sides  of  hills?  Why  are  mountain  tops  colder  than  lowlands?  What 
does  this  show  as  to  the  behavior  of  heat? 

Seasonal  Temperature  Range.  —  What  is  an  isotherm?  Why  are  iso- 
thermal lines  not  parallel  to  the  latitude  ?     What  is  the  normal  temper- 


456  PHYSICAL   GEOGRAPHY. 

ature  range?  How  is  this  shown  on  the  isothermal  charts?  What  do 
the  curves  show  ?  How  does  the  range  differ  in  various  places,  —  ocean, 
land,  and  different  latitudes  ?  Why  do  not  the  highest  parts  of  the 
curve  coincide  with  midsummer?  The  lowest  with  midwinter?  In 
what  ways  is  the  normal  curve  interfered  with? 

Isothermal  Charts.  —  Why  are  the  isotherms  of  the  southern  hemi- 
sphere more  regular  than  those  of  the  northern?  Why  is  the  heat 
equator  north  of  the  geographic  equator?  What  is  the  effect  of  the 
Gulf  Stream  ?  The  Labrador  current  ?  How  does  the  temperature 
distribution  of  the  west  coast  differ  from  that  of  the  east?  Why? 
Why  is  the  heat  equator  so  far  north  in  July  ?  Why  is  it  farther  north 
in  the  Atlantic  than  in  the  Pacific  ?  Why  is  the  deflecting  influence  of 
the  Gulf  Stream  greater  in  January  than  in  July  ?  Why  do  the  isother- 
mal lines  change  in  position  more  in  the  northern  than  in  the  southern 
hemisphere?  Where  are  the  coldest  places  on  the  earth  ?  Where  is  the 
cold  pole  ?  Where  are  the  greatest  seasonal  ranges  in  the  United  States  ? 
The  least?  Why?  Why  are  deserts  places  of  great  temperature  range? 
What  influence  of  topography  is  shown  on  the  chart  of  New  York  ? 

Daily  Temperature  Curve.  —  What  is  the  normal  daily  range  ?  When 
do  the  coldest  and  warmest  times  come?  Why?  How  does  the  curve 
differ  indifferent  places?  According  to  season?  By  accidental  inter- 
ruptions ? 

Temperature  Ranges.  —  How  closely  do  the  isotherms  give  the  real 
temperature  conditions?  Illustrate  by  San  Francisco.  Where  are  the 
lowest  and  highest  temperatures  found?  The  greatest  ranges?  Where 
are  the  greatest  and  least  ranges  in  the  United  States?  Give  an  example 
of  rapid  change.  Contrast  the  range  of  Key  West  and  Montana.  Give 
an  example  of  great  daily  temperature  range. 

Earth  Temperatures.  — What  is  the  normal  change  in  earth  tempera- 
ture? In  the  tropical  regions?  The  temperate?  The  arctic?  How 
does  the  temperature  of  the  surface  compare  with  that  of  the  air? 

CHAPTER  IV. 

General  Circulation  of  the  Atmosphere.    Pages  68-84. 

General  Statement.  —  Illustrate  mobility  of  the  air  by  its  action  on 
deserts.  Compare  with  the  effect  of  a  stove.  How  may  this  compari- 
son be  extended  to  the  atmospheric  circulation?      What  are  the  four 


QUESTIONS   UPON   THE  TEXT.  457 

principal  parts  to  this  circulation?  In  what  ways  are  these  changes 
registered  by  the  barometer  ?     What  is  a  barometric  gradient  ? 

Classification  of  the  Winds.  —  Give  the  classification  of  the  winds. 
What  are  the  planetary  or  permanent  winds  ? 

Planetary  or  Permanent  Winds:  Tiride  }Vi?ids. —  What  are  the  trade 
winds?  How  and  why  do  they  move?  Where  are  they  best  developed? 
Why  do  they  produce  deserts?  Why  do  they  often  cause  very  rainy 
belts?     How  can  the  same  wind  produce  these  two  opposite  effects? 

Doldruni  Belt.  —  What  are  the  doldrums?     Their  characteristics? 

Anti-trade  Winds.  —  In  what  direction  do  they  move?  How  do  we 
know  of  their  existence? 

Horse  Latitude  Winds.  —  Where  does  the  air  come  from?  What  are 
the  characteristics  of  the  belt? 

Prevailing  Westerlies.  —  What  is  the  circumpolar  whirl?  How  do  we 
know  the  permanency  of  these  winds  in  the  upper  air?  Of  what  value 
are  they  in  the  southern  hemisphere?    Why  not  also  in  the  northern? 

Periodical  Winds.  —  What  are  these? 

Seasonal  Winds.  —  Where  is  the  change  of  the  season  most  noticeable? 
What  effects  are  produced  in  the  atmospheric  circulation  near  the 
tropics?  What  is  the  seasonal  effect  on  the  land?  What  is  the  mon- 
soon? Where  are  monsoons  found?  How  is  their  influence  noticed 
in  the  United  States?  How  do  the  winds  of  Greenland  show  the 
influence  of  the  season  ?  What  is  the  effect  of  friction  between  wind 
and  land? 

Diurnal  Winds :  Sea  and  Land  Breezes.  —  What  is  the  cause  of  the  sea 
breeze?  When  does  it  come?  What  are  its  effects?  What  is  the  land 
breeze?  What  do  these  winds  resemble?  What  is  the  effect  of  the  sea 
breeze  in  the  trade-wind  belt?  What  is  the  general  effect  of  the  day- 
time heat  on  the  winds  of  the  land?     What  are  lake  breezes? 

Mountain  and  Valley  Breezes.  —  Describe  the  valley  breeze  as  to  cause 
and  effect.  The  mountain  breeze.  Why  are  the  former  more  violent 
than  the  latter?     Where  are  these  breezes  noticed  outside  of  mountains? 

Eclipse  and  Tidal  Breezes.  —  What  are  these? 

Irregular  Winds.  — How  do  they  differ  from  the  preceding? 

Accidental  Winds.  —  What  is  the  landslip  or  avalanche  blast?  What 
are  the  volcanic  winds?     The  waterfall  breeze? 

The  Nature  of  Winds.  —  What  is  the  real  nature  of  the  wind? 
What  causes  introduce  a  vertical  movement?  What  are  the  possible  uses 
of  the  internal  work  of  the  wind? 


458  PHYSICAL   GEOGEAPHY, 

CHAPTER  V. 
Storms.    Pages  85-106. 

Cyclonic  Storms.  —  What  is  a  storm?  What  are  some  of  the  causes  of 
storms?     AVhat  are  the  two  kinds  of  cyclonic  storms? 

Hurricanes  :  Description.  —  Where  do  the  hurricanes  begin  ?  The  ty- 
phoons? What  changes  are  noticed  as  the  storm  nears  and  passes  over  a 
place?    What  is  the  eye  of  a  storm?    How  is  the  air  moving  in  the  storm? 

Effects.  —  What  is  their  effect  upon  vessels?  Upon  the  coast?  State 
some  instances. 

Path.  —  What  is  the  natural  path  in  the  Xorth  Atlantic?  How  do 
they  sometimes  diverge  from  this?  What  is  their  path  in  the  Pacific? 
South  of  the  equator  ?     What  is  their  size  ?    Where  are  they  most  violent  ? 

Time  of  Occurrence.  —  When  are  they  most  common  in  the  northern 
hemisphere?    In  the  southern?    What  is  the  line  storm? 

Cause.  —  What  are  the  facts  to  be  accounted  for?  Why  may  we 
expect  that  the  heat  of  the  tropics  is  the  cause  for  their  beginning? 
What  would  account  for  the  whirling?  What  reason  is  there  for  the 
greater  influence  of  right-hand  deflection  in  certain  seasons?  Why 
should  they  be  confined  to  the  ocean?  What  is  the  effect  of  condensation 
of  water  vapor?  Why  do  the  storms  lose  energy  when  they  have  passed 
beyond  the  tropics?  What  is  the  explanation  of  the  path?  Describe  the 
hurricane.     State  its  cause  briefly  and  clearly. 

Temperate  Latitude  Cyclones :  Resemblance  to  Hurricanes.  —  How  do 
they  resemble  hurricanes? 

Differences  from  Hurricanes.  —  How  do  they  differ  in  general  behavior? 
In  time  and  place  of  development?    In  path?     What  is  the  usual  path? 

Effects.  —  Where  do  they  occur?  What  are  their  effects  in  the  United 
States  ? 

Winds.  —  How  do  these  vary?  What  changes  occur  as  the  storm 
passes?  What  is  the  sirocco?  The  foehn?  The  chinook?  The  bliz- 
zard?    The  norther? 

Anticyclones.  —  What  is  their  cause?  What  are  cold  waves?  What 
are  the  accompanying  conditions  of  winds? 

Cause.  —  What  was  the  former  theory?  What  objections  can  be  urged 
to  it?    State  a  possible  explanation.     What  is  the  reason  for  their  paths? 

Secondary  Storms:  Thunderstorms.  —  Where  do  they  occur?  Under 
what  conditions?     What  is  the  cause  for  the  thunder  cloud?    Its  form 


QUESTIONS   UPON  THE  TEXT.  459 

and  features?    What  is  their  relation  to  cyclonic  storms?     Their  path? 
What  is  a  cloud  burst?    Describe  and  discass  the  thunderstorm. 

Tornadoes  and  Waterspouts.  —  What  are  the  form  and  characteristic 
features  of  the  tornado?  Their  effects?  The  area  covered  and  time 
occupied  ?  In  what  respect  do  they  resemble  thunderstorms  ?  What  is 
the  cause?    What  is  a  waterspout? 

CHAPTER  VI. 
The  Moisture  of  the  Atmosphere.    Pages  107-123. 

Dew.  — What  is  the  cause  of  "sweat"  on  a  pitcher  of  ice  water?  How 
does  this  resemble  dew  formation?  At  what  temperature  and  time  will 
this  occur?  What  conditions  especially  favor  the  formation  of  dew? 
Why  does  dew  occur  more  readily  in  valleys  than  on  hilltops  ?  What  is 
the  main  cause  for  dew?    What  other  causes  also  aid? 

Frost.  —  What  is  frost  ?    What  prevents  it  ? 

Fog.  —  What  is  fog  ?  What  is  the  cause  for  ocean  fog  ?  What  is 
valley  fog?  In  what  other  ways  may  fog  be  caused?  What  is  the 
relation  of  dust  to  fog? 

Haze.  —  What  is  haze  ?    Its  cause  ? 

Mist.  —  What  is  mist? 

Clouds.  —  Of  what  are  clouds  composed?  Under  what  condition  are 
they  formed?  Give  the  classification  of  clouds.  Describe  the  cirrus ;  the 
cirro-stratus ;  cirro-cumulus ;  cumulus  ;  cumulo-stratus ;  stratus ;  nimbus. 

Rain.  —  What  is  the  cause  of  the  drop?  Under  what  conditions  is 
rain  caused?    What  relation  does  it  bear  to  clouds? 

Snow.  —  What  is  snow  ?     The  difference  between  snow  and  rain? 

Hail.  — What  is  hail? 

Distribution  of  Rainfall  in  the  World.  —  What  do  we  mean  by  rain- 
fall? Why  are  there  differences  according  to  altitude  and  latitude? 
What  is  the  cause  for  variation  in  tropical  regions  ?  What  is  the  effect 
of  steeply  rising  mountains?  What  are  the  two  main  causes  for  deserts? 
What  are  the  rainfall  peculiarities  within  the  belt  of  calms  ?  How  does 
the  rainfall  vary  from  coast  to  interior  ? 

Distribution  of  Rainfall  in  the  United  States. — What  are  the  causes 
for  the  heavy  rains  of  the  Texas  and  Florida  coasts?  For  the  differences 
between  the  east  and  west  coasts?  What  is  the  effect  of  the  high  west- 
ern mountains  upon  the  rainfall  of  the  western  half  of  the  country? 


460  PHYSICAL  GEOGBAPHT, 

Distribution  of  Snowfall.  —  Where  does  snow  fall  ?  Where  are  glaciers 
produced  ? 

Seasonal  Distribution  of  Rainfall.  —  What  is  the  e^'ect  of  the  migra- 
tion of  the  belt  of  calms?  How  do  the  monsoons  aifect  the  seasonal 
rainfall?  What  is  the  reason  for  the  winter  rains  of  Washington  and 
Oregon  ?     For  the  irregularities  of  rainfall  in  the  east  ? 

Irregularities  of  Rainfall.  —  What  is  the  normal  rainfall  ?  How  does  it 
sometimes  vary  from  this  ?    What  are  the  effects  of  heavy  downpours  ? 

CHAPTER  VII. 
Weather  and  Climate.    Pages  124-134. 

Weather.  —  What  is  weather  ?    Climate  ? 

Tropical  and  Arctic. —  What  are  the  weather  conditions  of  the  belt  of 
calms  ?    Of  the  trade-wind  belts  ?    Of  the  polar  regions  ? 

Temperate  Latitude  Weather.  —  What  are  the  weather  conditions  on 
the  northern  Pacific  coast  ?  In  the  mountains  east  of  this  ?  In  the 
deserts  between  the  mountains?  On  the  plains  of  Dakota,  etc.?  On 
the  more  southern  plains?  In  the  southern  coastal  states?  In  the 
northern  central  states?  What  is  the  cause  for  the  droughts?  What 
are  the  weather  conditions  of  the  northeastern  states?  What  are  the 
winter  conditions  in  this  belt?  The  summer  climate?  What  are  the 
typical  weather  conditions  in  temperate  latitudes?  How  do  those  de- 
scribed differ  in  Europe  ?    In  the  southern  hemisphere  ? 

Climate.  —  What  are  the  climatic  belts  ?    Their  subdivisions  ? 

Tropical  Climate.  —  What  is  the  general  climatic  condition?  The 
difference  between  the  ocean  and  the  land?  The  doldrum  and  trade- 
wind  belts?  What  are  the  differences  in  rainfall?  What  climatic 
peculiarities  are  caused  by  the  monsoon  condition  of  India? 

Temperate  Climate.  —  What  are  the  characteristics  of  the  climate  of 
this  belt?  What  are  its  subdivisions?  What  is  the  climate  of  the 
western  coasts  ?  Of  the  eastern  coasts  ?  The  interior  climate  ?  Of 
mountains?  Of  the  inter-montane  district.  State  the  climatic  differ- 
ences noticed  on  the  parallel  of  50°  N. 

Arctic  Climate.  —  What  are  its  characteristics? 

Minor  Variations.  —  What  are  some  of  these? 

Changes  in  Climate.  —  What  two  classes  of  evidence  point  to  climatic 
change?     What  is   the   supposed  thirty-six-year  cycle?    What   is   the 


QUESTIONS   UPON  THE  TEXT,  461 

geological  evidence   of  former   diiferences   in  climate?  What  recent 

geological  changes  are  recorded  in  the  United  States?  What  are  the 
possible  explanations  of  these  changes  ? 


CHAPTER  VIII. 
Geographic  Distribution  of  Animals  and  Plants.    Pages  135-148. 

General  Statement. —  AVhat  are  the  life  zones?  What  kinds  of  life 
occur  in  the  several  zones  ?  What  are  the  differences  between  the  life 
in  fresh  and  salt  water? 

The  Ocean.  —  What  causes  the  wide  distribution  of  ocean  life  ?  AVhat 
is  the  effect  of  temperature  on  distribution?  Where  in  the  ocean  are 
plants  unable  to  live?  Under  what  conditions  do  they  especially  thrive? 
What  is  the  difference  between  the  tropical  and  northern  animals? 

Fresh  "Water.  —  What  are  land-locked  animals?  What  forms  of  life 
are  found  in  fresh  water  ?    What  is  the  effect  of  change  to  salt  lake  ? 

The  Land  :  —  Effect  of  Temperature  and  Moisture.  —  What  is  the  effect 
of  temperature?  What  is  the  effect  of  arctic  cold  on  the  animals? 
On  the  plants?  Of  the  cold  of  high  temperate  latitudes?  What  is 
the  influence  of  altitude?  What  changes  in  vegetation  are  noticed  in 
ascending  high  mountains  ?  How  may  this  vary  on  the  opposite  sides 
of  a  mountain?    What  are  the  effects  of  aridity?    Of  great  moisture ? 

Plant  and  Animal  Habits.  —  How  do  the  seeds  effect  the  distribution  of 
plants  ?     What  animal  habits  influence  distribution  ? 

Life  Zones.  —  What  are  the  great  life  zones  and  their  subdivisions  ? 
How  do  the  continental  zones  resemble  one  another?  How  do  they 
differ  ?  What  do  these  differences  and  resemblances  show  ?  How  is  this 
illustrated  by  oceanic  islands?  In  the  Bermudas?  In  New  Zealand? 
The  East  Indies  ?     Australia  ? 

The  Spread  of  Life.  —  What  is  the  main  reason  for  the  distribution  of 
land  animals?  What  is  the  effect  of  the  winds  and  storms?  What 
animal  groups  are  distributed  by  this  means?  What  is  the  effect  of 
ocean  currents?  What  animals  are  thus  liable  to  be  carried?  Why  are 
large  animals  so  rare  on  oceanic  islands?  What  was  the  effect  of  the 
change  of  climate  causing  the  glacial  period? 

Barriers  to  the  Spread  of  Life.  —  What  is  the  great  barrier?  What 
does  Australia  teach  us  in  this  respect  ?    What  other  barriers  are  there  ? 

Effect  of  Man.  —  What  is  the  effect  ?    Ts  there  any  limit  to  it  ? 


462  PHYSICAL   GEOGRAPHT, 


CHAPTER   IX. 

Form  and  General  Characteristics  of  the  Ocean. 

Pages  151-173. 

Distribution  of  Land  and  Water.  —  What  are  the  main  features  of 
distribution  of  land  and  water  ? 

Composition  of  Ocean  Water.  —  What  are  the  principal  ingredients  of 
salt  water?  How  much  variation  is  there  in  salt  impurities?  What  are 
the  reasons  for  this  ? 

Color  and  Phosphorescence.  —  What  is  the  natural  color  of  the  ocean  ? 
Why?    Are  there  other  colors?     What  is  phosphorescence ? 

Exploration  of  the  Ocean  Bottom.  —  What  reasons  led  to  the  belief  that 
animals  could  not  live  here?  How  can  the  animals  exist  under  the 
great  pressure  ?    What  has  led  to  the  study  of  the  deep  sea  ? 

Methods  Used  in  Deep-sea  Explorations :  Sounding.  —  What  are  the 
objects  sought  ?  What  is  a  fathom  ?  Describe  the  sounding  machine. 
What  other  facfcs  are  learned  during  the  sounding? 

Dredging.  —  Describe  the  deep-sea  trawl.  How  correct  a  knowledge 
may  we  expect  to  obtain  by  dredging  ? 

Topography  of  the  Ocean  Bottom :  General.  —  What  is  the  fundamental 
difference  between  the  land  and  ocean  bottom  topography?  Why  are 
there  greater  occasional  elevations  in  the  ocean  ?  Why  greater  general 
levelness  ?  What  are  the  general  features  of  the  ocean  bottom  ?  State 
some  of  the  excessive  differences  in  elevation  in  the  ocean. 

The  Atlantic  Ocean.  —  What  is  the  continental  shelf?  The  continental 
slope  ?  The  oceanic  plateau  ?  The  mid-Atlantic  ridge  ?  What  are  the 
features  east  of  this  ?  What  features  are  shown  in  a  cross-section  of  the 
Atlantic  ? 

Other  Oceans.  —  How  do  the  features  of  the  Pacific  correspond  with 
those  of  the  Atlantic  ?  What  is  the  deepest  known  point  in  the  Pacific  ? 
In  the  Atlantic  ?     Compare  ocean  depths  with  land  elevations. 

Topography  near  the  Coast.  —  Compare  this  with  the  ocean  depths. 

Temperature  of  the  Ocean  Bottom.  —  What  are  the  temperature  features 
of  the  ocean  bottom  near  the  land  ?  How  does  this  change  with  increas- 
ing depth  ?  What  is  the  general  temperature  condition  of  the  waters  of 
the  ocean  bottom?  How  does  this  vary  in  such  places  as  the  Medi- 
terranean?    The  Gulf  of  Mexico?     What  is  the  explanation? 

Light  on  the  Ocean  Bottom.  —  What  is  the  probable  source  of  this  ? 


QUESTIONS   UPON   THE  TEXT.  463 

Materials  Composing  the  Ocean  Floor :  Mechanical  Sediments.  —  What 
are  the  two  sources  of  ocean  deposits  ? 

GloUgerina  Ooze.  —  What  is  this?  Where  does  it  occur?  How  is  it 
accumulated  ?    What  rock  resembles  it  ? 

Red  Clay.  —  What  is  this?  Where  does  it  occur?  What  materials 
compose  it  ?    How  large  an  area  does  it  cover  ? 

Life  in  the  Ocean  :  Pelagic  or  Surface  Faunas.  —  What  ocean  conditions 
especially  favor  abundant  life  ?  Why  is  the  temperature  uniform  ?  What 
conditions  favor  the  widespread  distribution  of  the  surface  animals? 
Under  what  conditions  do  they  live?  Do  animals  live  in  the  waters  be- 
tw^een  the  ocean  surface  and  bottom? 

Littoral  or  Shore  Faunas.  —  How  do  the  conditions  in  this  zone  resem- 
ble those  of  the  land?  What  is  the  effect  of  temperature  here?  Illus- 
trate. How  does  the  food  supply  influence  the  development  of  these 
animals?  Illustrate  by  coral  growth.  What  are  the  habits  among  shore- 
line animals  ?     How  do  these  vary  ? 

Faunas  of  the  Ocean  Bottom.  —  How  do  the  deep-sea  animals  show  the 
effect  of  pressure  when  brought  to  the  surface  ?  What  forms  live  on  the 
ocean  bottom?  What  is  the  main  cause  for  limiting  their  spread? 
Under  what  conditions  do  they  exist?  How  does  the  low  temperature 
tend  to  diminish  the  abundance  of  animals?  What  is  their  food  sup- 
ply ?  How  does  this  also  limit  their  abundance  ?  How  do  they  obtain 
their  oxygen  ?  What  do  they  prove  with  reference  to  oceanic  circulation  ? 
How  does  the  oxygen  supply  tend  to  limit  the  abundance  of  life  ? 

CHAPTER  X. 
Ocean  Waves  and  Currents.    Pages  174-191. 

Wind  Waves.  —  What  is  their  cause  ?  Their  form?  How  do  they 
move  ?  What  change  is  caused  at  the  shore?  How  far  do  they  extend? 
When  are  they  formed?  How  do  they  act  on  the  shore?  What  are 
their  effects  ?     Their  every-day  action  ?    How  may  their  effects  be  seen  ? 

Earthquake  Waves.  -—  What  are  these  ?  How  do  they  behave  ?  What 
are  their  important  effects  ?     How  far  may  they  travel  ? 

Storm  Waves.  —  What  causes  tend  to  produce  these  ?     Their  effect  ? 

Ocean  Surface  Temperatures.  —  What  is  the  natural  change  from  place 
to  place?  How  may  this  be  made  to  vary?  What  influence  is  noticed 
near  the  coast?     What  are  the  conditions  in  mid-ocean?     Why  is  the 


464  PHYSICAL   GEOGBAPHY, 

warm  surface  water  so  shallow  ?,  Why  are  the  surface  temperatures  so 
constant  ? 

Ocean  Currents :  Planetary  Circulation.  —  What  resemblance  is  there 
between  ocean  and  air  circulation  ?  What  reasons  are  there  for  believing 
in  a  planetary,  oceanic  circulation  ? 

The  System  of  Ocean  CwTents.  —  What  is  the  circulation  in  equatorial 
regions?  What  is  the  North  Atlantic  drift?  What  becomes  of  the 
water  entering  the  Caribbean?  What  is  the  origin  of  the  Gulf  Stream? 
Its  course  ?  What  is  the  Labrador  current  ?  Briefly  describe  the  general 
circulation  of  the  North  Atlantic.  What  are  the  conditions  in  the  South 
Atlantic?  What  is  the  circulation  of  the  North  Pacific?  What  is  the 
Kuro  Siwo  ?  What  is  the  circulation  of  the  South  Atlantic  ?  What  are 
the  main  features  of  the  oceanic  circulation  ? 

Cause  of  Ocean  Currents.  —  What  reasons  are  there  for  doubting  the 
temperature  theory?  What  is  the  apparent  explanation?  What  facts 
support  this  ?  What  influence  has  the  temperature  difference  ?  What 
causes  determine  the  course  of  currents  ?  What  would  be  the  circula- 
tion if  there  were  no  land? 

7'he  Gulf  Stream.  —  What  is  the  reason  for  its  warmth?  Its  velocity? 
How  does  it  vary  in  velocity? 

The  Labrador  Current.  —  What  is  its  course? 

Effects  of  Ocean  Currents.  —  What  is  the  most  important  effect? 
What  would  result  if  there  were  no  circulation?  What  indication  is 
there  of  an  important  influence  upon  temperature?  How  much  heat  is 
carried?  What  is  the  influence  upon  rainfall?  Upon  sailing  vessels? 
In  producing  fogs?     Upon  animal  life  in  the  ocean  ? 

CHAPTER    XI. 
Tides.    Pages  192-203. 

Nature  of  the  Tidal  Wave.  —  What  is  the  nature  of  the  wave? 

Cause  of  Tides.  —  W^hat  is  the  origin  of  the  wave?  Why  is  the  in- 
fluence of  the  moon  greater  than  that  of  the  sun? 

Effect  of  the  Land.  —  What  is  the  natural  course  of  the  wave?  What 
is  the  cause  for  its  peculiar  movement  in  the  Atlantic?  What  is  the 
change  introduced  in  bays  ?  What  are  the  peculiarities  near  the  British 
Isles  ?  In  the  approaches  to  New  York  ?  How  does  the  height  vary  ? 
How  may  it  be  lessened?    How  may  it  be  increased  ?    What  is  the  effect 


QUESTIONS   UPON   THE  TEXT,  465 

of  the  difference  in  the  height  of  the  tide  in  connected  bays?  AVhat  are 
tidal  races?    Illustrate.     What  is  the  tidal  bore? 

Other  Causes  for  Variation  in  Tidal  Height.  —  What  is  the  effect  of  the 
wind?  Of  air  pressure?  What  are  seiches?  How  does  the  relative 
position  of  sun  and  moon  influence  tidal  height?  What  are  spring 
tides?  Neap  tides?  What  is  the  influence  of  perigee  and  apogee? 
What  other  astronomic  causes  for  variation  are  there? 

Effects  of  Tides.  —  What  is  their  influence  upon  navigation?  In 
changing  the  coast?  What  is  their  effect  in  estuaries?  How  are  the 
tides  utilized? 


CHAPTER  Xn. 
The  Crust  of  the  Earth.    Pages  205-223. 

Interior  Conditions.  —  What  reasons  are  there  for  believing  that  the 
interior  of  the  earth  is  highly  heated?  What  was  the  former  belief? 
The  present  hypothesis?    What  is  the  apparent  effect  of  loss  of  heat? 

Movements  of  the  Crust.  —  What  classes  of  proofs  are  there  showing 
the  crust  to  be  in  movement  ?  State  some  of  the  historic  proof.  The 
geologic  evidence.     Is  this  a  movement  of  the  water  or  the  land  ? 

Disturbance  of  the  Rocks.  —  What  is  the  position  of  the  rocks  of  the 
crust?  By  what  means  are  they  changed  from  the  horizontal?  What 
is  a  monocline?  Anticline?  Syncline?  What  are  the  characteristics 
of  the  folds  in  mountains?  What  is  dip?  Strike?  A  fault?  A  fault- 
plane?    How  does  the  movement  take  place? 

Volcanic  Action. —  What  is  a  volcano?  A  lava  flow?  Volcanic  ash? 
Pumice?  How  do  volcanoes  vary  in  their  ejections?  How  large  an  area 
is  covered?     What  are  dykes?    Bosses? 

Rocks  of  the  Earth's  Crust.  —  What  are  the  three  groups  of  rocks? 
What  is  their  origin? 

Igneous  Rocks.  —  What  are  minerals?  What  rocks  are  crystalline? 
How  do  these  rocks  vary  chemically?  What  minerals  occur  in  them? 
Why  are  some  igneous  rocks  coarse  grained,  while  others  are  fine. 

Metamorphic  Rocks.  —  How  do  they  resemble  the  igneous?  What 
are  their  characteristics?  Their  origin?  What  are  the  common  rocks 
of  this  group? 

Sedimentary  Rocks.  —  What  are  the  three  subdivisions?     Which  is 

2h 


466  PHYSICAL   GEOGRAPHY. 

most  important?  How  are  the  mechanical  sediments  derived?  How 
are  they  accumulated  ?    What  are  the  kinds  ?     How  do  they  differ  ? 

Deposition  of  Sedimentary  Rocks.  —  In  what  position  are  they  depos- 
ited in  the  ocean?  What  is  the  origin  of  stratification?  What  are  the 
characteristic  deposits  in  the  sea?  What  are  the  characteristic  sedi- 
mentary rocks  on  the  land?  What  does  this  prove?  How  thick  are  the 
sediments.     What  does  this  prove?    What  is  an  unconformity? 

Consolidation  of  Sedimentary  Rocks.  —  How  are  rocks  cemented  ?  Illus- 
trate.    What  are  the  common  rock  cements? 

Geological  Chronology.  —  What  is  the  condition  of  the  rock  record? 
What  are  fossils?  How  has  a  record  of  early  life  been  obtained?  What 
does  this  show?  Can  the  age  be  told  by  fossils?  What  is  the  difference 
between  age  and  stage?  What  do  the  names  of  the  geological  periods 
really  indicate?  What  does  the  name  Carboniferous  mean?  Learn  the 
table  of  geological  ages.     The  groups  of  animals  that  lived  then. 

Age  of  the  Earth.  —  What  do  the  estimates  show?  What  does  geology 
show  as  to  the  age  of  the  earth  ?  Illustrate  by  Niagara  and  the  Colorado. 
By  volcanoes.  By  the  thickness  of  sedimentary  rocks.  What  are  the 
two  fundamental  conceptions  in  geology? 

CHAPTER  XIII. 
Denudation  of  the  Land.     Pages  224-248. 

Underground  Water.  —  How  does  water  find  its  way  into  the  rocks? 
How  does  it  move  through  them  ?  What  is  the  evidence  of  its  existence? 
How  is  it  able  to  dissolve?  What  evidence  of  this  is  there?  Why 
should  some  of  the  dissolved  mineral  substances  be  deposited?  What 
effects  are  produced  by  the  deposits  of  this  in  the  earth?  What  effect 
is  produced  by  underground  water  in  changing  minerals? 

The  Formation  of  Caverns.  —  What  is  their  origin?  What  are  stalac- 
tites?    Stalagmites?    What  is  the  origin  of  the  natural  bridge ? 

Springs  and  Artesian  Wells. — In  what  ways  are  springs  produced? 
What  are  the  conditions  favoring  the  accumulation  of  artesian  water? 
What  rock  is  particularly  favorable?  What  must  be  the  position  of  the 
rocks?  Why  does  the  water  rise  to  the  surface?  Why  does  it  not  rise 
above  the  permeable  layer?    What  is  the  use  of  this  water? 

Durability  of  Rocks. — How  do  rocks  vary  as  regards  durability  ?  What 
is  the  influence  of  texture?    What  is  meant  by  a  hard  rock? 


QUESTIONS    UPON   THE  TEXT,  467 

Weathering.  —  What  agents  are  engaged?  What  are  the  chemical 
changes?  How  do  these  affect  the  rocks?  In  what  rocks  are  they  most 
liable  to  act?  What  sedimentary  rocks  does  this  decay  form?  What 
is  the  most  important  mechanical  agent?  What  conditions  favor  the 
action  of  this  ?  Where  is  it  checked  ?  How  do  plants  aid  in  weather- 
ing? Animals?  How  widespread  is  the  action  of  weathering?  Where 
is  its  action  rapid?  .  Where  slow?  What  are  the  results?  With  what 
is  weathering  in  combat?  Which  has  excelled?  What  would  have 
been  the  result  had  there  been  no  re-elevations  ?  If  there  had  been  no 
other  agent  of  destruction  ?  What  agents  have  aided  the  effectiveness 
of  weathering?     What  is  residual  soil?     Where  is  it  important? 

Agents  of  Erosion.  —  What  are  the  most  important  of  these? 

Wind  Erosion.  —  Where  is  this  important?  What  is  its  effect  on  the 
seashore?  What  are  sand  dunes?  Why  is  wind  erosion  important  in 
arid  regions?     What  is  its  effect? 

Rain  Erosion.  —  When  does  this  action  commence ?  What  is  its  effect? 
Where  is  it  least  important  ?  What  is  the  origin  of  gravel  slopes  ?  What 
is  the  importance  of  gravity  ? 

Percolating  Water.  —  How  does  this  act  ?  How  does  it  act  mechani- 
cally?    How  are  avalanches  or  landslides  produced? 

River  Erosion.  —  What  tasks  are  rivers  engaged  in?  What  materials 
are  furnished  to  them?  How  do  these  materials  vary  in  amount  and 
kind?  In  what  way  does  the  river  erode?  Why  are  most  arid  land 
rivers  V-shaped?  Why  are  newly  begun  valleys  V-shaped?  What 
causes  them  to  broaden  ?  By  what  means  is  the  rate  of  erosion  caused 
to  vary?     How  do  rivers  vary?    What  is  their  most  important  office? 

Ocean  Erosion.  —  How  do  waves  act  ?  How  are  materials  removed  ? 
How  does  this  affect  the  coast  line. 

Glacial  Erosion.  —  How  does  ice  erosion  differ  from  that  of  water? 

Denudation.  —  What  is  denudation  ?  Whence  come  the  forces  ?  How 
do  the  agents  interact?     What  has  been  the  importance  of  their  action  ? 

CHAPTER  XIV. 

Topographic  Features  of  the  Earth's  Surface.  Pages  249-261. 

Continents  and  Ocean  Basins.  —  What  are  the  greater  irregularities  of 
the  earth?  What  is  the  arrangement  of  land  and  water?  What  is  the 
relative  size  of  the  continent  and  ocean  areas?  What  are  the  more 
important  features  of  the  ocean  bottom?    What  is  the  elevation  of  the 


468  PHYSICAL   GEOGRAPHY, 

land  compared  with  the  ocean  depth  ?  What  are  the  most  characteristic 
features  of  continents?  Are  the  continent  forms  permanent?  What 
changes  are  in  progress  ?     Where  is  the  real  continent  border  ? 

Physical  Geography  of  the  United  States.  —  What  are  the  five  geo- 
graphic provinces? 

Atlantic  Coast  Area.  —  What  is  the  extent  of  the  coast  plains?  What 
are  the  characteristics?  What  are  the  characteristics  of  the  plain  on 
the  landward  side  of  this?     Of  what  value  are  these  areas? 

The  Eastern  Mountains.  —  What  are  their  features?  What  are  the 
two  parts?  The  extent  of  the  older  mountains?  Their  features? 
What  is  the  relative  age  of  the  Appalachian  and  the  more  eastern  moun- 
tains? What  are  their  features?  Why  are  they  less  high  than  the 
Andes  and  Rockies?    What  are  their  most  important  mineral  products? 

The  Canadian  Highlands.  —  Where  do  these  extend  into  this  country? 

The  Central  Plains.  —  What  are  the  main  features  of  these  ?  Their 
extent  ?  How  are  they  interrupted  in  places  ?  For  what  are  they  valu- 
able ?    Why  are  they  not  forested  ? 

The  Cordilleran  Area.  —  What  are  its  main  features?  What  are  the 
features  on  the  eastern  base?  In  the  Rocky  Mountains?  West  of 
these?  In  the  Sierras?  At  the  western  base  of  these?  On  the  Pacific 
coast?  Why  are  these  mountains  so  high?  What  are  the  indications 
of  intense  denudation?  What  is  the  condition  of  volcanic  activity  in 
this  region?  Elsewhere  on  the  continent?  What  is  the  importance  of 
this  area  in  mineral  production  ? 

The  Drainage  of  the  Country.  —  (See  map.)  Into  what  oceans  does  the 
water  drain?  What  part  drains  to  the  Arctic?  Through  what  river? 
What  to  the  Pacific?  Through  what  large  rivers ?  What  two  important 
rivers  enter  the  Gulf  ?  What  is  the  condition  of  the  Appalachian  drain- 
age ?    What  are  the  features  of  the  St.  Lawrence  drainage? 

The  Shore  Line.  —  What  is  the  general  form  of  continents  ?  What  are 
the  main  features  of  the  Atlantic  coast  line  ?    Of  the  Pacific  ? 

CHAPTER  XV. 

River  Valleys.    Pages  262-284. 

General  Description.  —  What  is  a  river?  What  are  the  general  char- 
acteristics of  river  valleys  ?  What  is  a  river  system  ?  A  divide  ?  How 
do  rivers  differ?  What  was  the  former  belief  concerning  river  valleys  ? 
What  do  we  now  know  to  be  their  origin? 


QUIJSTIONS   UPON   THE  TEXT,  469 

Development  of  River  Valleys.  —  What  actions  combine  to  produce  the 
valley  ?  What  is  base  level  ?  When  in  river  development  does  erosion 
exceed  weathering  ?  When  does  this  cease  ?  What  would  be  the  ulti- 
mate result  ?  What  is  the  valley-form  in  youth  ?  In  maturity  ?  Where 
is  the  development  earliest  and  most  rapid  ?  How  may  the  valley-form 
vary  in  different  parts  of  the  course  ?  What  is  the  influence  of  rock 
structure?  Of  sediment  load?  Of  arid  conditions?  What  would  the 
canon  valley  show  as  to  age  ?  What  evidence  is  there  that  weathering  is 
in  progress?  What  other  features  of  youth  are  there?  How  do  the 
number  of  tributaries  show  age  ?  What  is  the  condition  of  the  divide  ? 
What  happens  when  vertical  erosion  ceases  ?  What  is  the  condition  of 
the  river  in  this  stage  of  maturity?  What  stage  have  most  valleys 
reached?  What  characteristic  features  have  led  to  the  division  of  the 
river  course  into  three  parts?  Why  cannot  this  be  considered  universal? 
How  may  the  rate  of  development  vary  ?  What  would  be  the  difference 
between  a  valley  on  a  plain  and  on  a  plateau  ?  How  may  the  climate 
influence  this?  Why  do  gorges  remain  so  long  in  mountains?  What 
would  be  the  effect  of  a  mountain  lake  ?  What  is  the  origin  of  the  broad 
valley  in  high  mountains? 

Adjustment  of  Streams.  —  What  is  a  consequent  stream  course?  How 
may  this  change  as  the  river  develops?    What  is  mature  adjustment? 

The  River  Divide.  —  Are  these  permanent  ?  How  may  they  change  ? 
What  is  the  law  of  monoclinal  shifting?  How  may  divides  be  suddenly 
changed  ? 

Accidents  to  Streams.  —  What  would  be  the  condition  if  no  accident 
interfered  with  river  development?  In  what  different  ways  do  these 
accidents  affect  stream  valleys  ?     What  are  composite  streams  ? 

Land  Movements.  —  What  are  the  three  kinds  ?  What  would  be  the 
effect  of  a  general  uplift?  Along  the  seashore?  Is  this  rejuvenation 
common  ?  What  would  be  the  effect  of  depression  ?  How  is  this  illus- 
trated on  the  eastern  coast?  How  will  folding  influence  the  streams? 
What  are  antecedent  rivers?  How  may  the  river  course  be  changed 
by  mountain  growth?    What  features  are  introduced? 

Climatic  Accidents.  — What  are  the  effects  of  a  change  to  a  condition 
of  dryness?  What  is  an  arroya?  What  are  withered  or  shrunken 
streams?  What  are  the  first  effects  of  glaciation?  How  are  the  lakes 
formed  along  the  margin  ?  Give  instances.  How  may  stream  courses 
be  changed  ?  What  are  the  results  ?  What  effects  are  produced  by  vol- 
canic action  ?    By  avalanches  ?    Why  is  the  old-age  stage  not  reached  ? 


470  PHYSICAL   GEOGRAPHY, 

CHAPTER  XVI. 
Deltas,  Floodplains,  Waterfalls,  and  Lakes.    Pages  285-305. 

Deltas.  —  Where  are  delta  deposits  made?  What  is  the  alluvial 
fan?  What  conditions  favor  delta  formation  in  the  ocean?  Why  are 
lakes  favorable  places  for  these?  How  does  the  river  flow  over  the 
delta?     What  are  distributaries?     How  does  the  delta  grow ? 

Floodplains.  —  Where  are  these  found?  What  causes  floodplains 
among  mountains?  What  is  the  most  common  cause  for  floodplains? 
How  may  they  merge  into  deltas  ?  What  effect  would  be  produced  by 
tilting  the  land?  From  changes  of  climate?  What  are  the  character- 
istics of  floodplains?  What  is  the  course  of  the  stream?  What  are 
oxbow  cut-offs?  How  are  the  floodplains  raised?  How  does  the  flood- 
plain  material  move  down  stream?  What  is  the  effect  of  the  floodplain 
upon  tributaries  ? 

Waterfalls.  —  What  is  their  origin?  What  cause  has  produced  most 
of  these  ?  What  was  the  origin  of  Niagara  ?  Its  history  ?  The  falls  of 
St.  Anthony?  What  other  causes  produce  falls?  What  is  the  fall  line? 
Its  importance?  How  may  waterfalls  be  naturally  developed?  What 
is  the  most  common  position  of  the  rocks  in  which  these  are  developed? 
What  is  the  origin  of  such  rapids  as  those  of  the  Colorado? 

Lakes.  —  How  do  they  differ?  What  relation  do  they  bear  to  rivers? 
How  may  they  be  produced  ?  What  is  the  most  common  cause  ?  What 
other  accidents  produce  lakes?  What  are  original  lakes?  How  may 
lakes  be  naturally  developed?  How  permanent  are  lakes?  How  are 
they  destroyed  ?  Which  of  the  processes  is  the  more  important  ?  Why  ? 
Under  what  conditions  may  cutting  at  the  outlet  become  of  importance  ? 
Illustrate  one  of  these  by  Niagara.  What  is  the  effect  of  evaporation  ? 
What  have  been  the  changes  in  the  Great  Basin? 

Swamps.  —  What  relation  do  these  bear  to  lakes?  How  does  the 
change  take  place?    In  what  other  ways  may  swamps  originate? 

CHAPTER  XVII. 
Glaciers.     Pages  306-327. 

Cause  of  Glaciers.  —  What  is  a  glacier?  How  does  it  form?  What 
determines  the  terminus?  Where  are  conditions  found  which  favor 
their  formation?     What  are  the  kinds  of  glaciers? 

Alpine  or  Valley  Glaciers.  —  Where  are  these  found?  What  is  the 
snow  field?     How  does  the  glacier  receive   its   supply?     How  does   it 


QUESTIONS   UPON  THE  TEXT.  471 

move?  "What  are  crevasses?  What  is  an  ice  fall ?  What  are  the  causes 
of  irregularities  on  the  surface  ?  How  is  the  glacier  supplied  with  rock 
material?  What  is  the  lateral  moraine?  The  medial  moraine?  The 
ground  moraine?  The  terminal  moraine?  What  is  the  origin  of  the  ice 
cave  ?  What  are  the  characteristic  features  of  the  valley  glacier?  What 
are  the  characteristics  of  the  glacier  at  the  foot  of  Mt.  St.  Elias. 

Continental  Glaciers.  —  Where  are  these  now  found?  How  extensive 
are  they  ?  How  thick  are  these  ice  sheets  ?  What  are  the  features  of 
the  Greenland  glacier  ?     What  are  nunataks  ? 

Icebergs.  —  What  is  floe  ice  ?  How  are  icebergs  formed  ?  How  far  do 
they  journey  ?     How  much  is  below  water  ?     How  high  are  some  bergs  ? 

Glacial  Period  :  Area  covered  by  Ice.  —  What  recent  changes  of  climate 
have  taken  place  ?  What  was  the  effect  ?  How  extensive  was  the  glacia- 
tion?  What  were  the  conditions  in  northeastern  America?  In  Europe? 
Were  these  two  areas  connected?  What  were  the  conditions  in  Asia? 
In  western  America  ?  What  do  we  know  about  the  cause  for  this  change 
in  climate?    How  long  ago  did  the  ice  sheet  disappear? 

Terminal  Moraine.  —  How^  did  the  glacier  resemble  the  Greenland  ice 
sheet?  What  was  accumulated  at  its  margin?  Where  is  the  terminal 
moraine?     What  are  its  features ? 

Formation  of  Soil.  —  What  is  till  or  boulder  clay?  What  are  its 
characteristics?  What  are  the  signs  of  a  scouring  action?  How  deep 
is  the  soil?    What  other  kinds  of  soil  were  left? 

Formation  of  Lakes.  —  How  were  temporary  lakes  formed?  What 
effect  was  produced  in  the  Red  River  valley  ?  What  was  the  size  and 
extent  of  this  lake?  What  is  the  proof  of  this?  How  were  lakes  formed 
by  the  deposit  of  glacial  drift  ?  How  were  rock  basins  formed  ?  What 
large  lakes  were  produced  by  the  action  of  the  glacier? 

Formation  of  Waterfalls.  —  How  were  the  stream  courses  interfered 
with  ?  Why  are  the  new  valleys  gorges  ?  Why  were  waterfalls  caused  ? 
What  was  the  general  effect  of  the  ice  upon  the  topography  ? 

CHAPTER  XVIII. 

The  Coast  Line.     Pages  328-349. 

General  Statement. — What  changes  are  taking  place?    What  agents 
are  at  work?     How  do  lake  and  sea  shores  resemble  one  another? 
Effect  of  Elevation.  —  What  are  the  effects  of  this  ? 
Effect  of  Depression.  —  What  are  the  effects  of  this?    What  would 


472  PHYSICAL   GEOGRAPHY. 

result  from  the  depression  of  the  land  bringing  sea  level  to  the  place 
occupied  by  the  student?  What  is  shown  on  the  coast  of  Maine? 
Where  else  is  this  also  shown?  What  are  the  two  general  types  of 
coast?    Why?     Give  illustrations. 

Effect  of  Sediment.  —  What  becomes  of  most  of  the  sediment?  When 
the  sediment  supply  is  too  great,  what  becomes  of  it  ?  Why  are  sand  bars 
produced  in  the  sea? 

Effect  of  Waves  and  Currents.  —  What  are  these  doing  on  exposed 
coasts?  Give  some  illustration  from  the  English  coast.  From  the 
American.  What  are  bars?  Spits?  Hooks?  How  does  the  effect  vary 
with  the  hardness  of  the  rock?  What  is  the  tendency  of  the  wave 
work?  How  are  lagoons  formed  by  beach  barriers?  What  is  the 
natural  form  of  the  beach  ? 

Effect  of  Plants.  —  What  is  the  effect  of  seaweeds  ?  Of  the  mangrove? 
Of  the  marsh  grasses  ? 

Effect  of  Animals.  —  Under  what  conditions  may  corals  live?  Why 
are  they  absent  from  some  tropical  coasts  ?  What  do  they  build?  What 
are  barrier  reefs?  Keys?  Atolls?  Why  are  these  above  sea  level? 
What  is  the  Darwin  theory  for  atolls. 

Changes  in  Coast  Form.  —  What  are  some  of  the  causes  for  change  ? 
What  are  some  of  the  recent  changes  on  the  eastern  coast  ? 

Islands.  —  How  do  these  vary?  What  are  the  classes?  What  are 
the  classes  of  oceanic  islands?  Where  are  these  represented  on  the 
coast?  What  are  the  causes  for  most  of  the  islands?  What  becomes  of 
islands  if  left  to  the  waves  ?    Illustrate. 

Promontories.  —  What  is  the  difference  between  capes  and  promonto- 
ries ?  What  are  the  causes  for  some  of  the  larger  promontories  ?  What 
is  the  origin  of  the  Nova  Scotia  peninsula  ?     Florida  ?     Sandy  Hook  ? 

Lake  Shores.  —  What  are  the  features  of  these?  How  are  capes  and 
islands  formed  in  them?  What  is  the  origin  of  the  Thousand  Islands? 
What  part  of  the  seashore  do  most  lake  shores  resemble  ? 

Fossil  Shore  Lines.  —  How  are  these  formed  ?  What  are  their  features  ? 
How  durable  are  they  ?    Give  some  instances  of  these. 

CHAPTER  XIX. 

Plateaus  and  Mountains.    Pages  350-369. 

Plateaus.  —  What  is  a  plateau?  How  does  it  differ  from  a  plain? 
With  what  are  they  associated?    Where  are  they  found?    What  large 


QUESTIONS   UPON   THE  TEXT,  473 

plateaus  are  covered  by  lava?  What  is  the  climate  of  the  plateaus  of 
the  west?  How  do  the  western  plains  differ  from  the  prairies ?  What  is 
the  condition  of  the  river  valleys?  Why?  What  is  the  characteristic 
topography  of  the  high  plateaus?    What  is  a  mesa?     Abutte? 

Mountains :  Characteristics  of  Mountains.  —  What  is  a  mountain  ? 
What  is  the  origin  of  the  features?  What  is  a  mountain  system?  A 
Cordillera?  A  range?  A  ridge?  How  do  they  resemble  one  another? 
What  is  a  peak  ?  What  is  the  origin  of  the  peak  ?  Of  what  are  they 
made  ?  How  do  they  differ  from  the  ridge  ?  What  other  kinds  of  peaks 
are  there?  What  are  hills  of  circumdenudation ?  What  are  interior 
basins  ?  Where  are  they  found  ?  What  is  their  comparative  importance 
in  different  continents  ?  What  is  the  origin  of  the  longitudinal  valleys  ? 
What  are  parks?  What  is  the  origin  of  mountain  gorges?  What  are 
passes?  What  is  the  characteristic  topography  in  mountains?  What 
are  the  reasons  for  this  ?  What  are  the  features  of  the  flora  ?  Why  are 
mountain  peaks  rugged?  Upon  what  does  the  form  of  the  peak,  ridge, 
etc.,  depend?     When  are  mountains  most  rugged? 

The  Origin  of  Mountains.  —  State  the  contraction  theory.  What  com- 
parison may  be  made  concerning  the  wrinkling  of  the  crust?  What 
is  the  value  of  this  theory?  What  is  the  history  of  mountain  folds? 
How  do  mountains  grow?  What  happens  as  they  grow?  What  would 
be  the  result  if  denudation  had  been  absent? 

Sculpturing  of  Mountains.  —  What  determines  the  result  of  this? 

The  Drainage  of  Mountains.  —  What  determines  the  drainage  ?  What 
are  the  characteristics  of  the  mountain  drainage?  What  are  longitudi- 
nal streams  ?  Transverse  valleys  ?  What  may  be  said  about  the  origin 
of  antecedent  valleys ?  What  is  the  origin  of  mountain  lakes?  Their 
characteristics  ? 

Destruction  of  Mountains.  —  What  are  the  features  of  young  moun- 
tains? Why?  What  happens  as  the  age  increases?  What  is  the  stage 
reached  by  the  Appalachians?  By  the  eastern  highlands?  What 
changes  occur  in  the  position  of  the  hard  and  soft  layers?  What  are 
synclinal  mountains  ? 

CHAPTER  XX. 

Volcanoes,  Earthquakes,  and  Geysers.    Pages  370-389. 

Volcanoes  :  Distribution.  —  Where  do  they  occur  with  reference  to  the 
sea?    To  mountains?    Where  found  in  North  America?    What  about 


474  PHYSICAL   GEOGRAPHY, 

their  former  abundance?  Have  they  occurred  in  all  parts  of  the 
world? 

Materials  Erupted.  —  What  substances  are  erupted  ?  What  is  the 
cause  of  pumice  ?  ^V'hat  are  the  effects  of  the  steam  ?  What  is  a  mud 
flow?  How  does  the  lava  flow  move?  What  is  the  extent  of  the  lava? 
How  does  this  differ  from  ash?    What  was  the  effect  of  Krakatoa? 

Eruptions  of  Volcanoes.  —  How  do  these  vary  as  to  violence  ?  Contrast 
the  eruption  of  Krakatoa  with  those  of  the  Lipari  Islands.  What  is  the 
case  in  Vesuvius  ?  In  the  Hawaiian  Islands  ?  What  kinds  of  volcanoes 
are  the  most  violent  ?    What  are  the  three  groups  ? 

Foi-m  of  Cone.  —  How  does  a  volcano  grow?  What  tends  to  destroy 
the  cone?  Where  are  they  steepest?  What  is  their  angle  of  slope? 
How  do  lava  and  ash  cones  differ  ? 

Effects  of  Volcanic  Eruptions.  —  What  are  the  more  important  effects  ? 

Extinct  Volcanoes.  —  What  happens  after  volcanoes  become  extinct  ? 
What  are  volcanic  necks  ?    AVhat  are  dykes  ?    What  are  buttes  ?     Mesas  ? 

Cause  of  Volcanoes.  —  What  is  the  immediate  cause?  What  is  the 
origin  of  the  heat?     What  is  the  association  with  mountains?     Why? 

Earthquakes. — Where  do  these  occur  ?  What  is  the  nature  of  the  shock  ? 
W^hat  is  the  focus  ?  The  epicentrum  ?  How  does  the  shock  travel  out 
from  the  center  ?     What  are  the  effects  ?    What  may  cause  earthquakes  ? 

Geysers  and  Hot  Springs.  —  What  is  the  origin  of  hot  springs  ?  With 
what  are  they  commonly  associated  ?  AVhat  is  the  association  with  ore 
deposits  ?  What  is  the  relation  between  geysers  and  hot  springs  ?  Where 
are  geysers  found  ?     What  are  their  characteristics  ? 

CHAPTER    XXL 

The  Topography  of  the  Land.     Pages  390-406. 

General  Statement.  —  How  are  land  forms  derived  ?  AVhat  are  the 
forces?  What  would  be  the  result  if  denudation  had  been  absent? 
What  are  the  opposing  forces  succeeding  in  accomplishing  ?  What  feat- 
ures and  forces  determine  the  complexity  of  the  land  form  ? 

Constructive  Land  Form  :  By  Internal  Forces.  —  How  are  these  compli- 
cated ?  What  are  the  larger  constructive  forms  ?  W^hat  is  the  origin  of 
the  coast  plains  ?    Volcanic  cones  ? 

By  Agents  of  Denudation.  —  What  constructive  forms  are  produced  by 
gravity?  By  wind?  In  lakes?  By  rivers?  By  glaciers?  In  the  ocean? 
How  are  these  forms  modified  ? 


QUESTIONS   UPON   THE  TEXT,  475 

By  Animal  and  Plant  Life.  —  State  some  of  these. 

Effect  of  Rock  Structure  upon  Topography.  —  How  may  rock  character- 
istics influence  the  action  of  denudation?  What  are  the  features  in  high 
mountains  ?  In  arid  climates  ?  What  influence  does  the  stage  of  devel- 
opment have  upon  topographic  form?  What  is  the  effect  of  uniformity 
of  texture?  Of  variation?  What  is  the  effect  of  position?  When  the 
rocks  are  horizontal?  What  are  terraces  of  differential  degradation? 
What  forms  result  when  the  rocks  dip  gently  ?  What  are  the  features 
found  in  traveling  over  such  a  region  ?  What  results  on  the  seacoast  ? 
AVhat  happens  in  mountains  with  steeply  inclined  strata?  When  rocks 
are  harder  than  others,  what  happens  ?  What  results  when  submergence 
occurs  ?    What  is  the  interaction  of  the  various  forces  ? 


CHAPTER    XXn. 
Man  and  Nature.    Pages  407-419. 

General  Statement.  —  How  does  man's  present  condition  differ  from 
that  of  the  past?    How  may  the  subject  be  divided? 

Modifying  Influence  of  Man.  —  State  some  of  the  ways  in  which  he 
modifies  nature.  How  is  he  modifying  animals  and  plants?  What  is 
his  influence  in  spreading  animals  and  plants?    In  destroying  them?     . 

Man  and  the  Forest.  —  What  is  the  effect  of  the  forest  covering  in  pro- 
tecting the  soil?  How  does  it  influence  the  distribution  of  rainfall? 
How  does  it  affect  the  streams?  State  briefly  the  importance  of  the 
forest.     What  reasons  are  there  for  thinking  that  it  affects  the  climate  ? 

Influence  of  Nature  upon  Man.  —  What  change  is  taking  place  ?  What 
differences  do  we  find  between  people  of  different  occupations  ?  How  do 
the  inhabitants  of  the  several  zones  differ  ?  What  was  the  former  condi- 
tion of  man?  How  did  the  surroundings  influence  the  Chinese?  The 
Egyptians?  The  inhabitants  of  the  Italian  peninsula?  Of  Greece? 
Why  was  the  Mediterranean  the  natural  seat  of  early  navigation?  How 
were  the  Northmen  influenced  by  surroundings?  The  English?  What 
are  the  reasons  for  the  large  number  of  European  nations?  What  is 
illustrated  by  Switzerland?  Why  is  America  so  different  from  Europe 
in  respect  to  political  divisions?  What  physical  features  aided  in  the 
discovery  of  America?  What  influence  did  this  discovery  exert?  Why 
were  the  American  settlements  made  near  the  coast?  What  was  the 
influence  of  the  forest  barrier?     Why  was  the  settlement  of  the  interior 


4Y6  PHYSICAL  GEOGHAPBT. 

delayed?  When  reached,  why  was  its  settlement  relatively  easy?  What 
caused  the  development  of  the  far  west?  What  has  determined  the 
position  of  the  towns  of  New  England?  What  relation  is  there  between 
the  industries  of  the  country  and  the  surroundings? 

CHAPTER    XXIII. 
Economic  Products  of  the  Earth.    Pages  420-430. 

Soil.  —  What  is  its  origin?     Its  value? 

Building  Stones.  —  What  is  the  origin  of  granite?  What  other  stones 
are  sold  as  granite?  What  are  the  metamorphic  building  stones?  What 
is  the  origin  of  slate?  Of  marble?  What  are  the  causes  of  metamorph- 
ism?  What  are  the  sedimentary  building  stones?  How  abundant  are 
they?  What  other  mineral  substances  are  used  for  building?  What  is 
the  origin  of  the  clay  deposits  ? 

Economic  Deposits  of  Sedimentary  Origin.  —  What  is  the  origin  of  the 
substances  used  for  grinding  and  polishing?  Of  rock  salt?  What  other 
substances  occur  with  it  ?     What  is  the  origin  of  the  fertilizers? 

Miscellaneous  Substances.  —  What  are  some  of  these  ? 

Coal.  —  What  evidence  is  there  pointing  to  the  origin  of  coal?  How 
may  coal  have  been  formed  by  drifting  wood?  By  accumulation  in  bogs? 
On  seashore  marshes  ?  What  is  the  probable  origin  of  coal  ?  How  is  the 
coal  changed?  What  are  these  changes?  In  what  periods  in  the  earth's 
history  has  coal  been  formed  ? 

Natural  Gas  and  Petroleum.  —  What  is  their  value?  How  do  thev 
occur?  How  constant  is  their  supply?  What  is  their  origin?  What 
artificial  products  do  they  resemble  ? 

Ore  Deposits.  —  In  what  associations  do  metals  occur  ?  How  must  they 
occur  to  be  profitable  ?  Describe  replacement  deposits.  Fissure  deposits. 
Sedimentary  deposits.  What  are  placer  deposits?  Where  do  they  occur? 
What  other  substances,  besides  gold,  occur  in  this  way? 

Distribution  of  Ore  Deposits.  —  Where  do  they  most  commonly  occur  ? 
Why  are  so  few  of  the  metals  produced  from  the  Cordilleras  ?  What  are 
the  reasons  for  the  importance  of  the  Cordilleras  ? 

Mineral  Wealth  of  the  United  States.  —  In  the  production  of  what 
metals  does  this  country  take  first  rank?  In  what  does  it  take  second 
rank  ?  How  valuable  is  the  industry,  and  how  is  it  distributed  ?  Which 
is  the  leading  state ?  Its  products?  The  second?  Its  products?  The 
third  ?     The  fourth  ?    What  has  been  the  value  of  this  great  wealth  ? 


INDEX. 


Absolute  humidity,  37,  434. 
Absorption  of  heat,  30 ;  of  light.  28 ',  of 

vapor,  36. 
Accidental  winds,  70,  82. 
Accidents  to  river  valleys,  275. 
Active  volcanoes,  377. 
Adirondacks,  256,  304,  409,  410 ;  lakes  in, 

299,  300 ;  peaks  of,  356. 
Adjustment  of  streams,  272. 
Aerial  life,  135. 
Age  of  earth,  218,  221. 
Ages,  geological,  220. 
Air,  effect  of  heat  upon,  68. 
Air  currents,  deflection  of,  by  rotation, 

39. 
Alaska,  glaciers  in,  308,  311,  312,  313. 
Algeria,  high  temperature  of,  63. 
Alkaline  plains,  394. 
Alluvial  fan,  285,  288. 
Alpine  glacier,  307. 
Alpine  snow  field,  306. 
Alps,  368;  glaciers  in,  308;  valleys  in, 

271,  272. 
Altitude,  effect  upon  temperature,  47. 
American  Falls,  Niagara,  295. 
Andromeda  nebula,  17. 
Anemometer,  433, 
Aneroid  barometer,  433. 
Animals,   aid  in  disintegrating    rocks, 

235;    effect  on  coast,  340;    habits  of, 

141,  142 ;  importance  in  ocean,  395 ;  of 

ocean  bottom,  156,  169,  171. 
Antarctic,  icebergs  in,  316 ;  ice  sheet  of, 

313. 
Antecedent  valleys,  278,  365. 
Anticline,  208,  209. 
Anticyclones,  100. 
Anti-trade  winds,  70,  74. 


Apogee,  13 ;  effect  of,  upon  tide,  200. 

Appalachian  Mountains,  255,  368. 

Arctic  climate,  132 

Arctic,  life  in,  138. 

Arctic  weather,  125. 

Argon  in  atmosphere,  24. 

Arid  land  drainage,  280 ;  vegetation,  141, 
142. 

Arroya,  279. 

Artemesia  geyser,  387. 

Artesian  wells,  229. 

Ash,  volcanic,  371,  373. 

Asia,  monsoons  of,  77. 

Asteroids,  6,  11. 

Atlantic,  249;  circulation  of,  72,  73; 
coast  plains,  254;  cross-section  of, 
158,  251 ;  temperature  of,  181 ;  tides 
of,  194 ;  topography  of  bottom, 
158;  volcanoes  in,  370;  winds  of,  72, 
73. 

Atmosphere,  5 ;  absorption  of  vapor  by, 
36 ;  circulation  of,  68 ;  composition  of, 
23;  cooling  of,  on  ascension,  33;  den- 
sity of,  23,  24 ;   effect  of  earth's  rota- 
tion on,  39;  effect  of  gravity  on,  39 
effect  of  heat  upon,  68 ;  extent  of,  23 
moisture    in,    35 ;    pressure    of,  39 
saturation  of,  36 ;  warming  of,  32,  33. 

Atmospheric  circulation,  parts  of,  69. 

Atmospheric  electricity,  29. 

Atmospheric  movements,  effect  of,  upon 
temperature,  44. 

Atolls,  342. 

Aurora,  29. 

Australia,  animals  of,  145;  monsoons 
of,  77. 

Avalanche  blast,  70,  82. 

Avalanches,  effect  upon  rivers,  282 ;  for- 
mation of,  241. 

Avalanche  lake,  N.Y.,  299. 


477 


478 


PHYSICAL   GEOGRAPHY. 


B. 


Bad  Lands,  S.D.,  247. 

Baker's  Park,  357. 

Bank  of  river,  262. 

Banner  cloud,  111. 

Barograph,  433. 

Barometer,  433 ;  change  during  passage 
of  hurricane,  86. 

Barometric  gradient, 

Barrier  reefs,  341. 

Bars,  331,  334,  335,  394,  395;  in  rivers, 
288. 

Base  level,  265. 

Basin  of  Minas,  tidal  flat  in,  202. 

Basin  Ranges,  258. 

Bay  of  Fundy,  tides  of,  196,  197. 

Bays,  origin  of,  276,  277,  329. 

Beaches,  335,  336,  395  ;  abandoned,  349. 

Bermudas,  depth  of  ocean  near,  158. 

Black-bulb  thermometer,  432. 

Blizzards,  100. 

Bonneville,  Lake,  302. 

Borax,  422. 

Bosses,  212,  383. 

Boulder  clay,  321. 

Boulders  in  moraine,  320,  321 ;  on  sea- 
coast,  336. 

Breakers  on  the  coast,  174,  175. 

Brines,  422. 

British  Isles,  tides  near,  194, 195. 

Bromine,  422. 

Building  stone,  420. 

Butte,  353,  356,  383,  402. 

Buzzard's  Bay,  tides  of,  197. 


C. 


Calm  belts,  migration  of,  70,  76. 

Campos,  122. 

Canadian  Highlands,  256. 

Canons,  240,  242,  267,  270. 

Canon  of  Colorado,  270,  352,  391. 

Cape  Ann,  Mass.,  coast  of,  203,  334-338, 
400,  401 ;  moraine  of,  320 ;  salt  marsh 
on,  339;  sand  dunes  on,  238. 

Cape  Cod,  Mass.,  sea  cliff,  328. 

Capes,  origin  of,  329,  345,  346. 

Carbonic  acid  gas  in  atmosphere,  24. 

Casco  Bay,  Me.,  islands  of,  345. 


Cave,  227 ;  river  source  in,  263. 

Caverns,  formation  of,  226. 

Cementing  of  rocks,  217. 

Centigrade  scale,  431. 

Central  plains,  256. 

Ceres,  11. 

Charleston  earthquake,  384. 

Chasms,  origin  of,  400. 

Chemical  deposits  from  underground 
water,  225. 

Cherrapunji,  rainfall  of,  122. 

Chesapeake  Bay,  origin  of,  278. 

Chicago,  lake  breeze  of,  80. 

Chili,  changes  of  level  in,  206. 

China,  loess  in,  399. 

Chinook  wind,  99. 

Chromosphere,  7. 

Chronology,  geological,  218. 

Circulation  of  atmosphere,  68 ;  of  ocean, 
182;  of  water  on  ocean  bottom,  163, 
172. 

Circumpolar  whirl,  75,  101. 

Cirro-cumulus  cloud,  113. 

Cirro-stratus  cloud,  113. 

Cirrus  cloud,  113. 

Clays,  421. 

Climate,  129;  of  arctic  zone,  132; 
changes  in,  132, 317 ;  effect  upon  lakes, 
302 ;  effect  on  man,  413 ;  effect  of  upon 
streams,  267,  279;  influence  upon 
topography,  398 ;  minor  variations  of, 
132;  of  plateaus,  351 ;  of  St.  Louis,  62 ; 
of  San  Francisco,  62;  study  of,  435; 
of  temperate  latitude,  130;  of  trop- 
ical regions,  130. 

Climatic  zones,  129. 

Cloudbursts,  104, 123. 

Clouds,  111;  kinds  of,  112;  study  of, 
434;  of  cyclonic  storms,  94. 

Coal,  423. 

Coast,  cause  of  irregularities  of,  330; 
changes  in,  343;  destruction  of,  332; 
effect  of  tides  on,  201;  effect  of  waves 
upon,  176. 

Coast  line,  328,  395;  of  United  States, 
261. 

Coast  Ranges,  259. 

Coastal  plain,  254,  393. 

Cold  pole,  56. 

Cold  waves,  51,  100,  127,  128. 


INDEX. 


479 


Color,  cause  of,  29 ;  of  ocean  water,  152. 

Colorado  canon,  270,  352,  391 ;  rapids  in, 
298. 

Colorado,  mineral  wealth  of,  430. 

Comets,  6,  15. 

Complex  valleys,  276. 

Composite  valleys,  276. 

Concretions,  427. 

Conduction  of  heat,  32. 

Cone  delta,  285. 

Cone  of  volcano,  378. 

Consequent  river  courses,  272. 

Constructional  land  forms,  392. 

Continental  glacier,  307,  313,  318. 

Continental  islands,  344. 

Continental  shelf,  158. 

Continental  slope,  159. 

Continents,  249 ;  cause  of,  390 ;  change 
in  form  of,  252 ;  features  of,  251. 

Contorted  limestone,  214. 

Contour  interval,  439. 

Contour  maps,  438. 

Contraction  theory,  363. 

Convection,  32. 

Coral  deposits,  395. 

Coral  islands,  395. 

Coral  keys,  341. 

Coral  reefs,  168,  341. 

Corals,  conditions  favoring  develop- 
ment of,  169 ;  effect  of  Gulf  Stream 
upon,  191 ;  effect  of  ocean  currents 
on,  342 ;  effect  of  temperature  on, 
136 ;  importance  on  coast,  340. 

Cordillera,  354. 

Cordilleras,  age  of,  259;  minerals  of, 
259 ;  ores  of,  428,  429 ;  volcanoes  of, 
371 ;  of  the  west,  257. 

Corona,  7,  28. 

Crater  of  geysers,  388;  of  volcanoes, 
379. 

Crevasse,  309. 

Crust  of  earth,  205 ;  movements  of,  206, 
390 ;  rocks  of,  212. 

Crystalline  rocks,  213. 

Cumberland  valley,  model  of,  437. 

Cumulo-stratus  clouds,  113,  114. 

Cumulus  cloud,  113. 

Currents  of  ocean,  163,  172,  182,  328; 
deflection  of,  by  earth's  rotation,  39 ; 
efl  3ct  of,  on  coast,  330,  332. 


Cyclonic  storms,  85. 


D. 


Daily  temperature  ranges,  43,  59,  60, 
61,  65. 

Day,  cause  of,  13. 

Dead  Sea,  life  in,  137. 

Death  Valley,  258. 

Deccan,  plateau  of,  351. 

Deep-sea  animals,  oxygen  supply  of,  171. 

Deep  sea,  circulation  of  water  in,  163, 
172 ;  dredging  of,  155 ;  exploration  of, 
153;  life  in,  169;  light  in,  163;  sedi- 
ments of,  164;  sounding  of,  154; 
sounding  machine,  154;  temperature 
of,  162 ;  trawl,  154,  155. 

Deforesting  Adirondacks,  410. 

Delaware  Bay,  origin  of,  277. 

Delta  lakes,  300. 

Delta  of  Mississippi,  344. 

Deltas,  285, 331, 394 ;  conditions  favoring 
formation  of,  286 ;  in  lakes,  287 ;  rela- 
■tion  to  floodplain,  288;  rivers  on,  287. 

Denudation,  246,  390,  393,  395-397;  ab- 
sence of  effects  of,  on  ocean  floor,  156 ; 
of  land,  224 ;  of  mountains,  360,  361, 
364,  367  ;  of  volcanoes,  379. 

Depression,  effect  on  coast,  329. 

Depth  of  ocean,  157, 158. 

Desert  dust  whirl,  68. 

Deserts,  cause  of,  74, 117 ;  life  in,  141. 

Dew,  107. 

Dew  point,  37. 

Diathermanous  bodies,  30. 

Diffusion  of  light,  26. 

Dip  of  rocks,  209;  relation  of  topog- 
raphy to,  402. 

Dismal  Swamp,  304,  425. 

Dissection  of  valleys,  278. 

Distributaries  on  deltas,  287. 

Diurnal  winds,  70,  76,  79. 

Diversion  of  streams  by  mountain 
growth,  278. 

Divide,  263 ;  changes  in,  273. 

Doldrums,  climate  of,  130;  density  of 
ocean  in,  152  ;  migration  of,  70,  76, 122 ; 
rains  of,  117 ;  thunderstorms  of,  102  j 
weather  of,  124. 

Donati's  comet,  15. 


480 


PHYSICAL   GEOGRAPHY, 


Dormant  volcanoes,  377. 

Drainage  of  mountains,  365. 

Dredging,  155. 

Droughts,  cause  of,  126. 

Drowned  rivers,  276,  277. 

Dust,  effect  of,  on  light,  26,  27 ;  in  at- 
mosphere, 24;  importance  in  forma- 
tion of  fog,  110. 

Dust  whirl  of  the  desert,  68. 

Dykes,  212,  383. 

E. 

Earth,  11;  age  of,  218,  221;  condition 
of,  11;  elevations  on  the  surface  of, 
4;  form  of,  3;  interior  condition  of, 
11,  205;  irregularities  on  surface,  3; 
movements   of,    12,    33;    movements 
of    surface,  206;    revolution   of,   12; 
rotation  of,  13 ;  water  on  the  surface 
of,  4. 
Earth  columns,  232. 
Earth  temperature,  65. 
Earthquake  waves,  178. 
Earthquakes,  383 ;  association  with  vol- 
canoes, 381 ;  cause  of,  385. 
Eastern  mountains,  254. 
Eastport,  Me.,  tides  at,  199,  200. 
Ebb  of  tide,  192. 
Eclipse  breezes,  70,  76,  82. 
Electricity,  29. 

Elevation,  effect  on  coast,  329. 
Elk  Mountains,  Col.,  354. 
English  Channel,  tides  of,  194,  195. 
Epicentrum  of  earthquakes,  384. 
Erosion,  agents  of,  238 ;  by  glaciers,  245 ; 
by  oceanic  forces,  244,  328 ;   by  rain, 
239 ;  by  rivers,  241,  265,  268 ;  by  under- 
ground water,  240;  of  volcanoes,  379; 
by  wind,  238. 
Eruption  of  geysers,  389. 
Estuary,  filling  with  salt  marsh,  339. 
Estuaries,  origin  of,  276,  277,  329. 
Eurasia,  map  of,  250. 
Europe,  glaciation  of,  318. 
Evaporation  of  water,  31,  35,  39,  120; 

measurement  of,  434. 
Extinct    lakes,    302;     volcanoes,     378, 

381. 
Eye  of  storm,  87,  96. 


P. 

Fahrenheit  scale,  431. 

Fall  line,  296. 

Fan  delta,  285. 

Fathom,  154. 

Fault,  210. 

Fault  plane,  210. 

Faults,  association  of  earthquakes  with, 

386 ;  relation  of  ores  to,  428. 
Faunas  of  ocean  bottom,  169 ;  of  ocean 

surface,  166. 
Fertilizers,  422. 
Finger  Lakes,  origin  of,  325. 
Floe  ice,  315. 
Floodplains,    288,    394 ;    characteristics 

of,  291 ;  building  of,  293. 
Floods,  influence  of  forest  upon,  410. 
Florida,  growth  of,  347;  keys  of,  341; 

lakes  of,  300;  swamps  of,  303,  424. 
Flow  of  tide,  192. 
Focus  of  earthquakes,  384. 
Foehn  wind,  99. 
Fog,  109. 

Food  of  ocean  animals,  168. 
Food  supply,  effect  on  ocean  life,  171. 
Forest  in  Adirondacks,  409. 
Forest,  importance  of,  409 ;  influence  on 

development  of  United  States,  417; 

influence  of  man  upon,  409 ;  on  moun- 
tains, 360. 
Forest  litter,  410. 
Fossils,  value  of,  219. 
Fresh  water  life,  137. 
Frost,  108;  action  on  mountain  peaks, 

361 ;  aid  in  disintegrating  rocks,  234. 
Fusiyama,  Japan,  380. 


G. 

Ganges  delta,  effect  of  hurricane  on,  89. 

Garden  of  Gods,  Col.,  231. 

Gas,  425. 

Gassendi,  lunar  crater  of,  14. 

Gay  Head,  retreat  of,  333. 

Geographic  distribution  of  animals  and 

plants,  135. 
Geological  ages,  220. 
Geological  chronology,  218. 
Geysers,  386,  387. 


INDEX. 


481 


Glacial  deposits,  394 ;  formation  of  lakes 

by,  299 ;  production  of  waterfalls  by, 

294. 
Glacial  erosion,  245. 
Glacial  lakes,  299,  317,  323. 
Glacial  period,  133,  316 ;  effect  upon  life, 

146 ;  effect  upon  streams,  280 ;  time  of, 

319. 
Glacial  scratches,  322. 
Glacial  soil,  321. 
Glaciers,  Alpine,  307 ;  in  Antarctic,  313, 

316;   cause  of,  122,  306;   continental, 

307,  313,  318 ;  effect  upon  valleys,  280 ; 

in  Greenland,  313,  314 ;  Piedmont,  313; 

relation  to  swamps,  304. 
Globigerina  ooze,  164;  area  of  deposit, 

166. 
Gneisses,  420. 
Gold  deposits,  428. 
Gorges,  caused  by  glacial  action,  281; 

formation  of,  242,  325 ;  in  mountains, 

271,  272,  358,  365;  near  Ithaca,  N.Y., 

215,  265. 
Graham's  Island,  346. 
Granite,  420;  disintegration  of ,  233. 
Gravity,  aid  in  erosion,  240;   effect  on 

atmosphere,  39. 
Great  Barrier  Reef,  Australia,  341. 
Great  Basin,  258,  281,  357;  drainage  of, 

279;  mountains  of,  258;  temperature 

of,  56. 
Great  Lakes,  effect  of  ice  on,  281 ;  origin 

of,  325 ;  winds  of,  80. 
Great  Salt  Lake,  303 ;  former  extension 

of,  281. 
Greenland,  glaciers  of,  313,  314;  winds 

of,  78. 
Green  River,  Utah,  278. 
Ground  moraine,  312. 
Gulf  of  Mexico,  temperature  of  bottom, 

163. 
Gnlf  Stream,  183,  187;  effect  on  corals, 
191, 347 ;  effect  on  life,  167, 168 ;  effect 
on  temperature,  51,  55,  190;  map  of, 
188 ;  velocity  of,  187. 
Gulf  weed,  136. 
Gypsum,  422. 

H. 
Hachure  maps,  437,  438. 


Hail,  116. 

Halo,  28. 

Harbors,  sea  action  in,  334. 

Hawaiian  Islands,  volcanoes  of,  377, 
380. 

Haze,  110. 

Heat,  30 ;  absorption  of,  30 ;  distribution 
of,  43;  effect  of  movements  of  the 
earth  upon,  33 ;  effect  upon  air,  68. 

Heat  equator,  53,  55. 

Heat  lightning,  30. 

Heligoland,  destruction  of,  332. 

Hell  Gate,  tide  at,  196,  198. 

Herculaneum,  destruction  of,  376. 

High  pressure,  433. 

Hills  of  circumdenudation,  356. 

Himalayas,  110,  368. 

Hooks,  333,  334,  347.  •    . 

Horse  latitude  winds,  70,  75. 

Hot  springs,  386. 

Humidity,  absolute,  37 ;  relative,  37 ; 
measurement  of,  434;  variation  in,  38. 

Hurricane,  86;  cause  of,  91;  cause  of 
path  of,  93;  destruction  caused  by, 
89 ;  difference  from  temperate  latitude 
cyclones,  95;  effects  of,  88;  features 
of,  87;  importance  of  vapor  in,  92; 
paths  of,  89, 90,  97 ;  pressure  in,  86, 88 ; 
reason  for  absence  from  South  Atlan- 
tic, 92 ;  reason  for  development  over 
ocean,  92;  resemblance  to  temperate 
latitude  cyclones,  93 ;  size  of,  90 ;  time 
of  occurrence  of,  91;  violence  of,  90; 
winds  of,  87,  88. 

Hygrometer,  434.  • 


I. 


Icebergs,  314-316. 

Ice  cave,  312. 

Ice  fall,  309. 

Igneous  rocks,  213,  420 ;  relation  of  ores 
to,  428. 

India,  monsoon  of,  77. 

Indianola,  Tex.,  destruction  by  hurri- 
cane, 89. 

Interior  basins,  356. 

Intruded  rocks,  212,  213. 

Irregular  winds,  70,  82. 

Island  life,  145. 


2i 


482 


PHYSICAL   GEOGBAPHY. 


Islands,  destruction  of,  346;  origin  of, 

329,  344 ;  volcanic,  244. 
Isothermal  charts,  51. 
Isotherms,  51 ;  of  New  York,  56,  59 ;  of 

United  States,  54,  56-58;  relation  to 

climate,  62. 
Ithaca,  N.Y.,  change  in  harometer  at, 

86 ;  cold  wave  at,  127, 128 ;  gorges  near, 

265,  297;    humidity  changes  in,  37; 

temperature  changes  in,  61, 66 ;  valley 

breeze  at,  81 ;  waterfall  near,  297. 


Japan,  earthquake  in,  385-387. 
Japanese  current,  184 ;  effect  on  temper- 
ature, 190. 
Jupiter,  9. 


Key  West,  temperature  of,  53, 56,  63, 65. 

Keys,  341. 

Krakatoa,  eruption  of,  374,  375,  380,  381, 

385. 
Kurile  Islands,  depth  of  ocean  near,  160, 
Kuro  Siwo,  184. 


L. 


Labrador  current,  189 ;  effect  upon  tem- 
perature, 53,  55,  168. 

Lagoon,  348. 

Lake  Agassiz,  324. 

Lake  Bonneville,  302. 

Lake  breeze,  80. 

Lake  Cham  plain,  origin  of,  325. 

Lake  Drummond,  origin  of,  300. 

Lake  Erie,  destruction  of,  by  Niagara, 
301. 

Lake  spit,  333. 

Lakes,  298,  394;  caused  by  beach  bar- 
riers, 335,  348;  caused  by  lava,  374, 
381 ;  deltas  in,  287 ;  destruction  of, 
300 ;  extinct,  302 ;  on  floodplains,  292 ; 
glacial,  281,  317, 323 ;  in  Adirondacks, 
409 ;  in  mountains,  366,  367 ;  in  young 
valleys,  269 ;  relation  to  swamps,  303 ; 
shores  of,  328, 348.  394. 

Land  breeze,  70,  79. 


Land,  denudation  of,  224 ;  effect  on  tem- 
perature, 55 ;  effect  on  tide,  193 ;  ele- 
vation of,  206;  life,  135,  137;  move- 
ment, effect  on  coast,  329;  topography 
of,  390. 

Land-locked  animals,  137. 

Landslide,  formation  of,  241. 

Landslip  blast,  70,  82. 

Latent  heat,  31,  35,  39. 

Lateral  moraine,  310,  312. 

Lava,  371. 

Lava  flow,  211,  372. 

Lava  plateaus,  351. 

Lawrence,  Mass.,  tornado,  105. 

Levees,  293. 

Life,  barriers  to  the  spread  of,  146 ;  de- 
struction by  volcanic  eruption,  381 ; 
effect  of  man  upon,  146, 147 ;  effect  of 
ocean  currents  on,  191 ;  of  the  air,  135 ; 
of  the  arctic  zones,  138;  of  the  dead 
seas,  137 ;  of  the  deserts,  141 ;  of  the 
fresh  water,  137;  of  the  land,  135, 
137;  of  the  mountains,  140;  of  the 
ocean,  135,  166;  of  the  ocean  bottom, 
153,  156,  169;  of  the  ocean  bottom, 
oxygen  supply  of,  171 ;  of  the  ocean 
shore,  167;  of  the  temperate  zones, 
139;  spread  of,  145. 

Life  zones,  135,  143;  of  United  States, 
144. 

Light,  25 ;  absorption  of,  28 ;  diffusion 
of,  26;  effect  of  dust  on,  26,  27;  on 
ocean  bottom,  163;  reflection  of,  27; 
refraction  of,  27 ;  selective  scattering 
of,  26;  source  of,  25. 

Lightning  in  thunderstorms,  29,  102, 
104. 

Limestone,  421. 

Line  storm,  91. 

Lipari  Islands,  volcanoes  of,  375. 

Littoral  faunas,  167. 

Llanos,  122. 

Loess  in  China,  399. 

Longitudinal  valleys  in  mountains,  358, 
365. 

Long's  Peak,  Col.,  360. 

Looming,  27. 

Low  pressure,  433. 

Low-pressure  areas,  tracks  of,  95-97. 

Lunar  craters,  15. 


INDEX. 


483 


M. 


Magnetic  pole,  29. 

Magnetism,  29. 

Malaspina  glacier,  313. 

Mammoth  hot  springs,  225. 

Man  and  nature,  407. 

Man  and  the  forest,  409. 

Man,  effect  in  distributing  life,  146, 147 ; 

modifying  influence  of,  407. 
Mangrove,  338. 
Mangrove  swamps,  424. 
Marble,  421. 
Mars,  9. 

Marsh  grass,  339. 
Massachusetts,  lakes  in,  324. 
Massachusetts  Bay,  tides  of,  197. 
Mato  Tepee,  Wyo.,  383. 
Matterhorn,  355. 

Mature  adjustment  of  streams,  273. 
Mature  river  valleys,  266,  267. 
Maximum  temperature  in  United  States, 

64. 
Maximum  thermometer,  432. 
Mechanical  sediments  in  ocean,  164. 
Medial  moraine,  310,  312. 
Mediterranean,   temperature  of    water 

in,  162,  163;  tides  of ,  197. 
Mercury,  8. 
Mesas,  353,  383. 

Metals,  426 ;  of  Cordilleras,  259. 
Metamorphic  rocks,  213,  214,  420. 
Meteorites,  16. 
Meteors,  6,  15, 16. 
Michigan,  mineral  wealth  of,  429. 
Mid-Atlantic  ridge,  159. 
Mineral  waters,  423. 
Minerals,  213,  426;  of  the  Cordilleras, 

259;  disintegration  of,  233;  effect  of 

water  upon,  226  ;   of  United   States, 

429. 
Minimum  temperatures  in  United  States, 

63. 
Minimum  thermometer,  432. 
Minnesota,  lakes  in,  324. 
Mirage,  27. 
Mississippi,  delta  of,  286, 344 ;  floodplain 

of,  290,  291,  292. 
Mississippi  valley  plains,  256. 
Mist,  111. 


Mitchell's  Peak,  height  of,  256. 

Models,  437. 

Moisture,  effect  upon  life,  141;  in  the 
atmosphere,  35 ;  measurement  of,  434. 

Monocline,  208. 

Monoclinal  shifting,  274. 

Monsoon  winds,  70,  77 ;  effect  upon  cli- 
mate, 130  ;  effect  upon  rainfall,  122. 

Montana,  mineral  wealth  of,  429 ;  tem- 
perature changes  in,  56,  64,  65. 

Monte  Somma,  376,  380. 

Moon,  13 ;  effect  in  producing  tide,  192, 
19^)-201. 

Moqui  Pueblo,  N.M.,  239. 

Moraines,  310,  312,  317,  319,  394. 

Mount  Dana,  glaciers  on,  310. 

Mount  Desert,  Me.,  411 ;  coast  of,  330. 

Mount  Everest,  110. 

Mount  Hood,  378. 

Mount  of  Holy  Cross,  Col.,  359. 

Mount  Marcy,  height  of,  256. 

Mount  St.  Elias,  139 ;  glaciers  of,  312. 

Mount  Shasta,  382  ;  glaciers  on,  307. 

Mountain  breeze,  70,  80. 

Mountain  gorges,  358. 

Mountain  thunderstorms,  102. 

Mountain  valleys,  271,  356. 

Mountain  vegetation,  140. 

Mountains,  association  of  volcanoes 
with,  370;  association  with  plateaus, 
350 ;  cause  of,  390 ;  characteristics  of, 
353 ;  in  continents,  251 ;  denudation 
of,  39(5, 404 ;  destruction  of,  367 ;  drain- 
age of,  365 ;  of  eastern  United  States, 
254 ;  effect  of  growth  of,  upon  streams, 
278;  effect  upon  temperature,  48; 
floodplains  among,  289;  glaciers  in, 
307;  of  Great  Basin,  258;  growth  of, 
363,  367 ;  life  in,  140 ;  origin  of,  362, 
393;  ruggedness  of,  361,  396;  sculp- 
turing of,  364 ;  valleys  in,  262-265 ;  of 
the  west,  257. 

Mud  flow,  372. 

Muir's  Butte,  Cal.,  379. 

N. 

Natural  bridge,  origin  of,  228. 
Natural  gas,  425. 
Natural  soda,  422. 


^ 


484 


PHYSICAL    GEOGRAPHY. 


Nature  and  man,  407. 

Nature,  influence  upon  man,  412. 

Navajo  Church,  Arizona,  397. 

Neap  tides,  200. 

Nebulae,  17,  21. 

Nebular  hypothesis,19;  verification  of  ,20. 

Neptune,  10. 

New  York,  isotherms  of,  56, 59 ;  tempera- 
ture of,  51. 

New  York  harbor,  tides  of,  194. 

New  Zealand,  animals  of,  145. 

Niagara,  effect  in  draining  Lake  Erie,  301. 

Niagara  Falls,  264,  295, 301 ;  age  of,  222 ; 
history  of,  294 ;  origin  of,  298. 

Night,  cause  of,  13. 

Nimbus  clouds,  113,  114. 

Nitrogen  in  atmosphere,  23, 

North  America,  cross-section  of,  251; 
glacier  of,  318 ;  shore  line  of,  261. 

North  Atlantic  drift,  183. 

Northeast  storms,  94. 

Norther,  100. 

Nunatak,  314. 


O. 


Oblong  geyser,  388. 

Occupation,  relation  to  topography,  418. 

Ocean,  area  of,  4,  151 ;  deposits  in,  395; 
depth  of,  160,  161 ;  effect  in  checking 
spread  of  life,  146 ;  effect  of,  on  tem- 
perature, 45;  erosion  in,  244,  328; 
phosphorescence  in,  152,  164;  shores 
of,  328;  surface  temperature  of,  179; 
volume  of,  4 ;  volcanoes  in,  370. 

Ocean  basins,  249. 

Ocean  bottom,  circulation  of  water  on, 
163, 172 ;  dredging  of,  155 ;  exploration 
of,  153,  156;  life  on,  153,  166,  169; 
light  on,  163 ;  sediments  of,  164 ;  tem- 
perature of,  155,162;  topography  of, 
156,  1(50,  250. 

Ocean  currents,  182;  cause  of,  185; 
cause  of  course,  187;  effects  of,  189; 
effect  on  life,  146, 166 ;  effect  on  tem- 
perature, 46,  180;  on  ocean  bottom, 
163,  172 ;  system  of,  183. 

Ocean  water,  color  of,  152;  composi- 
tion of,  151 ;  density  of,  152. 

Oceanic  islands,  244,  344. 


Oceanic  life,   135,  166;    habits  of,  169; 

influence  of  temperature  upon,  136. 
Oceanic  plateau,  157,  159. 
Oil,  425. 

Old  Faithful  Geyser,  389. 
Opaque  bodies,  29. 
Ore  deposits,  426. 

Oxbow  cut-off  lakes,  266,  292,  300. 
Oxygen,  in  atmosjihere,  23;  supply  of, 

to  deep-sea  animals,  171. 


Pacific  Ocean,  249;  topography  of  bot- 
tom, 160;  volcanoes  in,  370. 

Parks  in  mountains,  357,  358. 

Passes  in  mountains,  359. 

Path  of  storms,  89,  94-97. 

Peaks  in  mountains,  355. 

Peaks,  origin  of,  404. 

Peat  bogs,  304,  425. 

Pecos  River  valley,  N.M.,  plain  of,  350. 

Pelagic  faunas,  166. 

Pennsylvania,  mineral  wealth  of,  429, 

Percolating  water,  importance   of,  240. 

Perigee,  14 ;  effect  upon  tide,  200. 

Periodical  winds,  70,  76. 

Permanent  winds,  70,  71. 

Petroleum,  425. 

Phosphates,  422. 

Phosphorescence  in  ocean,  152,  164. 

Photosphere,  7. 

Piedmont  glacier,  313, 

Pike's  Peak,  355, 

Placer  deposits,  428. 

Plains,  350;  of  Atlantic  coast,  254;  in 
continents,  251 ;  of  Far  West,  351 ;  of 
Mississippi  valley,  256 ;  origin  of,  393 ; 
of  Red  River  valley,  394. 

Planetary  circulation  in  ocean,  182. 

Planetary  winds,  70,  71. 

Planets,  6,  8 ;  relative  distance  of,  5,  8 ; 
relative  size  of,  9, 

Plants,  aid  in  disintegrating  rocks,  234; 
effect  of,  on  coast,  337 ;  habits  of,  141 ; 
in  the  ocean,  136,  395, 

Plateau,  350;  association  with  moun- 
tains, 350;  of  continents,  251;  of  ice, 
314;  of  Mississippi  valley,  256;  of 
ocean  bottom,  157,  159,  250. 


INDEX, 


485 


Platinum,  428. 

Pompeii,  destruction  of,  372,  376. 

Porto  Rico,  depth  of  ocean  near,  157, 

160. 
Prairie  soil,  323. 
Prairies,  257,  351,  394. 
Pressure  of  atmospliere,  39. 
Pressure  in  hurricane,  88. 
Pressure,  measurement  of,  432 ;  relation 

to  winds,  70. 
Prevailing  westerlies,  70,  75, 
Promontories,  origin  of,  329,  345,  346. 
Psychrometer,  434. 
Pulpit  terrace,  225. 
Pumice,  211,  371. 


R. 


Radiant  energy,  30 ;  effect  upon  water, 
31 ;  effect  upon  the  land,  31 ;  passage 
through  the  atmosphere,  31 ;  reflection 
of,  30. 

Radiation  from  the  earth,  32. 

Rafe's  Chasm,  400. 

Rain,  cause  of,  114. 

Rain  erosion,  239. 

Rain  gauge,  435. 

Rain  in  thunderstorm,  104. 

Rainbow,  cause  of,  28. 

Rainfall,  distribution  of,  117;  in  dol- 
drum  belt,  74 ;  effect  of  forest  on,  412 ; 
irregularities  of,  123;  measurement 
of,  435 ;  seasonal  distribution  of,  122 ; 
in  trade-wind  belt,  74;  of  the  United 
States,  118. 

Ranges  of  mountains,  354. 

Rapids,  relation  to  waterfalls,  294. 

Ray  Brook,  Adirondacks,  304. 

Red  clay,  165. 

Red  River  valley,  effect  of  ice  on,  281; 
lake  in,  324 ;  plains  of,  350,  394. 

Red  Sea,  cause  of  color  of,  152. 

Reefs,  coral,  341. 

Reflection  of  radiant  energy,  30. 

Refraction  of  light,  27. 

Rejuvenation  of  river  valleys,  276. 

Relative  humidity,  37,  434. 

Replacement  deposit,  427. 

Residual  soil,  238. 

Revived  rivers,  276. 


Revolution,  effect  of,  upon  temperature, 
33. 

Rhone  glacier,  308. 

Ridges,  mountain,  354,  361,  368,  405. 

Right-hand  deflection,  40. 

Rio  Grande  valley  canon,  142 ;  talus  in, 
236. 

River  bank,  262. 

Rivers,  boulders  in  bed  of,  243;  acci- 
dents to,  275;  characteristics  of,  263; 
deposits  by,  394;  divide  of,  273;  effect 
of  forest  on,  410;  erosion  of,  241,  243; 
on  floodplains,  291 ;  at  margin  of  ice, 
312,  322 ;  in  mountains,  365 ;  relation 
of  lakes  to,  299 ;  sediment  in,  241 ;  of 
United  States,  259,  260. 

River  system,  263. 

River  valleys,  262 ;  adjustment  of,  272 ; 
drowned  by  sea,  330 ;  development  of, 
265;  difference  in  rate  of  develop- 
ment of,  270;  effect  of  climate  on, 
279 ;  origin  of,  264 ;  variation  among, 
244. 

Rock  basins,  325. 

Rock  pillars,  231. 

Rock  salt,  422. 

Rocks,  consolidation  of  sedimentary, 
217;  deposition  of  sedimentary,  215; 
disintegration  of,  233;  disturbance  of, 
207;  durability  of,  231;  of  earth's 
crust,  212 ;  elevation  of,  216 ;  horizon- 
tal, 208;  igneous,  213;  influence  of,  on 
form  of  crust,  334;  influence  upon 
stream  course,  272;  influence  upon 
topography,  208,  395,  402-405;  in- 
truded, 212,  213;  metamorphic,  213, 
214;  of  mountains,  355,  362;  sedimen- 
tary, 213,  214. 

Rocky  Mountains,  257,  368. 

Rotation,  deflective  effect  of,  39;  effect 
of,  on  temperature,  33. 

Royal  Gorge,  Col.,  265. 


S. 


St.  Anthony,  Falls  of,  296. 
St.  Louis,  temperature  of,  62. 
Salt  lakes,  302. 
Salt  marsh,  332,  339. 
Salts  in  the  ocean,  151. 


486 


PHYSICAL   GEOGRAPHY, 


Samoan  Islands,  hurricane  of,  88. 

San  Francisco,  temperature  of,  62. 

Sand  bars,  331. 

Sand  dunes,  239,  394. 

Sands,  421. 

Sandstone,  421. 

Sargasso  Sea,  13G,  167. 

Satellites,  6. 

Saturation  of  atmosphere,  36. 

Saturn,  10. 

Sea  breeze,  45,  70,  79. 

Sea  caves,  334,  335,  400. 

Sea  clifiEs,  347,  400,  401,  403;  Cape  Cod, 
Mass.,  328;  retreat  of,  332. 

Seasonal  temperature  range,  43,  48,  49, 
51. 

Seasonal  winds,  70,  76. 

Seasons,  12, 13,  33. 

Seaweeds,  importance  on  coast,  337, 338. 

Secondary  storms,  101. 

Sediment,  effect  of,  on  coast,  330;  on 
ocean  bottom,  164;  in  rivers,  241. 

Sedimentary  rocks,  213,  214,  330,  421 ; 
consolidation  of,  217 ;  deposition  of,  215. 

Seeds,  aid  in  distribution  of  plants,  141. 

Seiches,  198. 

Selective  scattering,  26. 

Shastina,  382. 

Shooting  stars,  15,  16. 

Shore  faunas,  167. 

Shore  lines,  328,  395 ;  above  sea  level, 
207 ;  change  in,  343, 400 ;  effect  of  tide 
on,  201;  fossil,  349;  of  lakes,  318;  of 
United  States,  261. 

Shrunken  streams,  279. 

Siberia,  low  temperature  of,  56,  63. 

Sierra  Nevada  Mountains,  258. 

Signal  Butte,  402. 

Sigsbee  deep-sea  sounding  machine,  154. 

Silver  deposits,  428. 

Sink-holes,  226. 

Sirocco  wind,  99. 

Slate,  421. 

Small  planets,  11. 

Snake  River  valley,  lava  plateau  of,  351, 
373. 

Snow,  115. 

Snowfall,  distribution  of,  121 ;  measure- 
ment of,  435. 

Snow  field,  306,  308. 


Snowflakes,  115. 

Snow  line,  139,  140. 

Soil,  420;  effect  of  forest  on,  441;  for- 
mation of,  237 ;  glacial,  321. 

Solar  light,  25. 

Solar  system,  5;  symmetry  of,  18. 

Sounding,  153. 

Sphagnum  moss,  304. 

Spits,  333,  334.  347,  394. 

Spring  tide,  199. 

Springs,  effect  of  forest  on,  410 ;  origin 
of,  228. 

Stalactites,  227. 

Stalagmites,  227. 

Stars,  17,  18. 

Steam  in  volcanoes,  372,  383. 

Stellar  system,  17. 

Storms,  85;  conditions  in,  88,  94;  of 
secondary  origin,  101 ;  tracks  of,  89, 
94-97 ;  waves  accompanying,  177, 
179 ;  winds  of,  70,  82,  85,  94,  98. 

Straits,  origin  of,  276,  277. 

Strata,  216;  influence  on  topography, 
401-405 ;  in  mountains,  364. 

Stratification,  216. 

Stratified  rocks,  215. 

Stratus  clouds,  113,  114. 

Stream  gold,  428. 

Strike,  209. 

Summer,  temperature  of,  50. 

Sun,  6;  effect  in  producing  tide,  193, 
199,  201 ;  movements  of,  8. 

Sun  spots,  8. 

Sunset  colors,  26. 

Surface  faunas  in  ocean,  l(i6. 

Swamps,  303,  394 ;  of  Florida,  424,  425 ; 
of  glacial  origin,  281,  283;  mangrove. 
339. 

Sweden,  changes  of  level  in,  206. 

Syncline,  208. 

Synclinal  mountains,  369. 

System  of  mountains,  354. 


T. 


Talus,  236,  240,  354. 
Taughannock  Falls,  294. 
Temperate  climate,  130. 
Temperate  latitude  cyclones,  86 ;  cause 
of,  100 ;  cause  of  path  of,  101 ;  differ- 


INDEX. 


487 


ence  from  hurricanes,  95 ;  effects  of, 
98 ;  features  of ,  9i ;  path  of,  97 ;  rela- 
tion of,  to  thunderstorms,  103 ;  resem- 
blance to  hurricanes,  93 ;  size  of,  96 ; 
time  of  occurrence  of,  96;  winds  of, 
9i,  98. 

Temperate  latitude,  weather  of,  125. 

Temperate  zone,  life  in,  139. 

Temperature,  of  Atlantic,  181 ;  daily 
ranges  in,  59,  65 ;  in  cold  wave,  127, 
128;  of  earth,  65,  205;  effect  of  alti- 
tude upon,  47 ;  effect  of  atmospheric 
movements  upon,  44;  effect  of  land 
upon,  55,  56,  57 ;  effect  upon  land 
life,  137 ;  effect  upon  mountain  life, 
140;  effect  of  mountains  upon,  48; 
effect  of  ocean  upon,  45;  effect  of 
ocean  currents  on,  46, 189 ;  effect  upon 
ocean  life,  136;  effect  of  sea  breeze 
on,  79,  80;  effect  of  topography  upon, 
47,  56;  of  Great  Basin,  56;  of  Key 
West,  53, 55,  56;  maximum,  in  United 
States,  64;  measurement  of,  431;  of 
midsummer,  50;  of  midwinter,  50; 
minimum,  in  United  States,  63 ;  ranges 
in,  61, 62, 64 ;  seasonal  range  of,  35, 43, 
48 ;  of  St.  Louis,  62 ;  of  San  Francisco, 
62;  of  United  States,  53;  variation 
of,  35,  43,51,60,  61. 

Temperature  of  ocean,  180;  effect  on 
circulation,  182,  185;  effect  on  life, 
166, 168,  170. 

Temperature  of  ocean  bottom,  155,  162, 
170. 

Temperature  of  ocean  surface,  179, 181. 

Terminal  moraine,  310,  312,  319. 

Terraces,  323,  400. 

Texas,  bars  on  coast  of,  331 ;  monsoons 
of,  78;  temperature  changes  in,  65. 

Thermograph,  432. 

Thermometer,  431. 

Thermometer  shelter,  432. 

Thibet,  temperature  ranges  in,  65. 

Thousand  Islands,  origin  of,  348. 

Thunder,  30,  102,  104. 

Thunderstorms,  101-103. 

Tidal  action  in  ocean,  328. 

Tidal  bore,  198. 

Tidal  breezes,  70,  76,  82. 

Tidal  currents,  importance  of,  201,  333. 


Tidal  height,  causes  for  variation  in,  193- 
203. 

Tidal  flat,  Basin  of  Minas,  202. 

Tidal  races,  198. 

Tidal  wave,  192. 

Tide-power,  uses  of,  202. 

Tides,  cause  of,  192;  effects  of,  201; 
effect  of  coast  upon,  193-198;  in  Eng- 
lish Channel,  194,  195;  in  New  York 
harbor,  194. 

Till,  321,  394. 

Timber  line  in  mountains,  138,  140,  359, 
360. 

Tin,  428. 

Topographic  maps,  437. 

Topography,  intiuence  upon  climate, 
47,56;  influence  on  man,  413-419;  of 
bottom  of  Atlantic  Ocean,  158;  of 
glaciated  regions,  320, 326 ;  of  the  land, 
390;  of  ocean  bottom,  156,  160,  161, 
250 ;  relation  to  rock  structure,  395. 

Tornadoes,  104. 

Trade-wind  belt,  70,  71 ;  climate  of,  130; 
effect  on  oceanic  circulation,  186; 
rain  caused  by,  117 ;  weather  in,  124. 

Translucent  bodies,  29. 

Transparent  bodies,  28. 

Transverse  mountain  valleys,  365. 

Trawl,  deep-sea,  155. 

Tributaries  of  river,  263,  269 ;  on  flood- 
plains,  293. 

Tropical  climate,  130. 

Tropical  cyclones,  86. 

Tropical  forest,  143. 

Tropical  weather,  124. 

Typhoons,  86. 


Unconformity,  217. 

Underground  water,  224,  233,  240,  386. 

Undulatory  theory,  25. 

United  States,  drainage  of,  259,  260; 
evaporation  in,  120;  ice  sheet  of, 
318;  isotherms  of,  53,  54,  56-58;  life 
zones  of,  144 ;  maximum  temperature 
of,  64 ;  mineral  wealth  of,  429 ;  mini- 
mum temperature  in,  63:  monsoon 
tendency  in,  78 ;  ores  of,  428;  physical 
geography  of,  253;  rainfall  of,  118, 
119,  122;  shore  line  of,  261;  temper- 


488 


PHYSICAL   GEOGBAPHY. 


ature  ranges  in,  62,  64;  terminal  mo- 
raine of,  320;  volcanoes  in,  259,  371. 
Uranus,  10. 


Valley  breeze,  70,  80. 

Valley  fog,  109. 

Valley  glaciers,  307 ;  former  extension 
of,  317. 

Valley  sides,  262. 

Valleys,  development  of,  242,  262,  267, 
270;  effect  of  climate  on,  279;  effect 
of  land  movements  on,  276 ;  in  moun- 
tains, 356. 

Vapor,  absorption  of,  36 ;  importance  of, 
in  hurricanes,  92;  variation  in  amount, 
36. 

Vegetation,  in  arid  land,  141,  142;  in 
mountains,  140 ;  in  swamps,  303. 

Veins,  427. 

Venus,  9. 

Vesuvius,  372,  376,  380. 

Vineyard  Sound,  tides  of,  197. 

Volcanic  action,  211. 

Volcanic  ash,  211,  371,  373. 

Volcanic  cone,  form  of,  378. 

Volcanic  island,  244. 

Volcanic  necks,  382,  383. 

Volcanic  winds,  70,  83. 

Volcanoes,  association  with  atolls,  343 
association  of  earthquakes  with,  385 
association  of  hot  springs  with,  387 
association  with  ores,  428;  cause  of, 
383;   destruction  of,  in  sea,  316;  dis- 
tribution of,  370 ;  effect  of  eruptions, 
381 ;  effect  upon  rivers,  282 ;  eruptions 
of,  374 ;   extinct,  378,  381 ;    materials 
erupted  by,  211,  371 ;   in  ocean,  156 ; 
origin  of,  393 ;  of  United  States,  259. 

Vulcano,  375. 

W. 

Water,  area  of,  on  earth,  151 ;  effect 
upon  rocks,  231,  233;  importance  in 
volcanoes,  383 ;  underground,  224,  240. 

Water  vapor  in  atmosphere,  24. 

Waterfall  breeze,  70,  83. 

Waterfalls,  268,  281,  294,  297,  325. 

Water  parting,  263. 

Waterspout,  106. 


Waterspout  waves,  179. 

Watkins  Glen,  N.Y.,  326. 

Waves,  174 ;  action  of,  on  coast,  176, 244, 
328,  330,  332;  cause  of,  176;  earth- 
quake, 178,  385 ;  form  of,  175 ;  storm, 
179. 

Weather,  124;  arctic,  125;  temperate 
latitude,  125;  tropical,  124;  study  of, 
435. 

Weather  maps,  435. 

Weather  predictions,  435. 

Weathering,  233 ;  effects  of,  236 ;  impor- 
tance of,  235,  265;  of  volcanoes, 
379. 

Westfield  River,  Mass.,  243. 

White  glacier,  Alaska,  311. 

White  Mountains,  N.H.,  356. 

Whitney  gl«,cier,  307. 

Wind  vane,  433. 

Wind  waves,  174. 

Winds,  accidental,  70,  82 ;  action  of,  393 ; 
aid  in  causing  rain,  117;  aid  in  distri- 
bution of  animals,  145  ;  of  Atlantic,  72, 
73 ;  classification  of,  70 ;  in  cold  wave, 
127,  128;  diurnal,  70,  76,  79;  effect 
upon  height  of  tide,  198;  effect  upon 
temperature,  44;  erosion  by,  238;  in 
the  general  circulation,  69;  of  horse 
latitude  belt,  75 ;  of  hurricane,  87,  88 ; 
internal  work  of,  83 ;  irregular,  70,  82 ; 
irregularities  of,  83;  measurement  of, 
433;  migration  of,  76;  monsoon,  70, 
77;  nature  of,  83;  periodical,  70,  76; 
permanent,  70,  71;  planetary,  70,  71; 
seasonal,  70,  76;  of  storm,  85,  94;  of 
temperate  latitude  cyclones,  94,  98; 
of  temperate  latitudes,  75 ;  in  thunder- 
storms, 103;  of  the  tornado,  105;  ver- 
tical movement  in,  83. 

Winter,  temperature  of,  50. 

Winter  thaws,  cause  of,  127. 

Withered  streams,  279. 


Yellow  Sea,  cause  of  color  of,  152. 

Yellowstone  Falls,  293. 

Yellowstone  Park,  geysers  of,  387-389. 

Yellowstone  Valley,  242,  268. 

Yosemite,  296,  398. 

Youth  in  river  valleys,  266. 


ECONOMIC  GEOLOGY 


OF  THE 


UNITED  STATES, 

WITH   BRIEFER  MENTION   OF   FOREIGN  MINERAL  PRODUCTS. 

By  RALPH   S.  TARR,  B.S.,  F.G.S.A., 

Assistant  Professor  of  Geology  at  Cornell  University. 

Second  Edition.    Revised.    $3.50. 


COMMENTS. 


t( ' 


'  I  am  more  than  pleased  with  your  new  *  Economic  Geology  of  the  United 
States.'  An  introduction  to  this  subject,  fully  abreast  of  its  recent  progress,  and 
especially  adapted  to  American  students  and  readers,  has  been  a  desideratum.  The 
book  is  admirably  suited  for  class  use,  and  I  shall  adopt  it  as  the  text-book  for  instruc- 
tion in  Economic  Geology  in  Colorado  College.  It  is  essentially  accurate,  while 
written  in  a  pleasant  and  popular  style,  and  is  one  of  the  few  books  on  practical 
geology  that  the  general  public  is  sure  to  pronounce  readable.  The  large  share  of 
attention  given  to  non-metallic  resources  is  an  especially  valuable  feature."  —  Francis 
W.  Cragin,  Professor  of  Geology,  Mineralogy,  and  Paleontology  at  Colorado 
College. 

"I  have  examined  Professor  R.  S.  Tarr's  'Economic  Geology'  with  much 
pleasure.  It  fills  a  felt  want.  It  will  be  found  not  only  very  helpful  to  students  and 
teachers  by  furnishing  the  fundamental  facts  of  the  science,  but  it  places  within  easy 
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plete statistics  of  our  national  resources.  The  numerous  tables  bringing  out  in  an 
analytic  way  the  comparative  resources  and  productiveness  of  our  country  and  of 
different  states,  are  a  specially  convenient  and  admirable  feature.  The  work  is  an 
interesting  demonstration  of  the  great  public  importance  of  the  science  of  geology." 
—  James  E.  Todd,  State  Geologist,  South  Dakota. 

"  It  is  one  of  those  books  that  is  valuable  for  what  it  omits,  and  for  the  concise 
method  of  presenting  its  data.  The  American  engineer  has  now  the  ability  to  acquire 
the  latest  knowledge  of  the  theories,  locations,  and  statistics  of  the  leading  American 
ore  bodies  at  a  glance.  Were  my  course  one  of  text-books,  I  should  certainly  use  it, 
and  I  have  already  called  the  attention  of  my  students  to  its  value  as  a  book  of 
reference."  —  Edward  H.  Williams,  Professor  of  Mining,  Engineering,  and 
Geology  at  Lehigh  University. 

"I  have  taken  time  for  a  careful  examination  of  the  work;  and  it  gives  me 
pleasure  to  say  that  it  is  very  satisfactory.  Regarded  simply  as  a  general  treatise 
on  Economic  Geology,  it  is  a  distinct  advance  on  anything  that  we  had  before;  while 
in  its  relations  to  the  Economic  deposits  of  this  country  it  is  almost  a  new  creation 
and  certainly  supplies  a  want  long  and  keenly  felt  by  both  teachers  and  general 
students.  Its  appearance  was  most  timely  in  my  case,  and  my  class  in  Economic 
Geology  are  already  using  it  as  a  text-book."  —  William  O.  Crosby,  Assistant 
Professor  of  Structural  and  Economic  Geology  at  the  Massachusetts  Institute  of 
Technology. 


MACMILLAN    &    CO., 

66   FIFTH   AVENUE,    NEW   YORK. 


By  SIR  ARCHIBALD  GEIKIE,  F.R.S.,  LL.D., 

Director-General  of  the  Geological  Surveys  of  the  United 

Kingdom. 

THE  TEACHING  OF  GEOGRAPHY. 

SUGGESTIONS  REGARDING    PRINCIPLES   AND  METHODS 
FOR    THE    USE    OF  TEACHERS. 

Second  Edition.    Cloth.     i6mo.    60  cents. 


"  Since  Dr.  Geikie,  following  the  suggestions  of  the  Germans  or  such  geographers 
as  Guyot,  has  developed  this  rational  doctrine  in  teaching  geography,  many  writers 
and  teachers  have  adopted  the  new  methods,  and,  as  yet,  we  know  of  no  wiser  or 
more  suggestive  work  for  teachers  of  geography  than  Dr.  Geikie's. 

"  Any  change  in  old  forms  or  customs  is  looked  upon  askance.  Even  in  the 
methods  of  teaching,  reforms  are  slow,  and  should  be.  Our  author  says:  '  Inveter- 
ate habits  of  use  and  wont  are  apt  to  blind  us  to  the  need  of  change,  and  any  attempt 
to  alter  the  existing  system  touches  many  kinds  of  vested  interests.  Even  those  who 
sympathize  with  the  proposals  for  reform  raise  their  hands  in  despair  and  ask  where, 
amid  the  crowds  of  subjects  now  demanded,  room  is  to  be  opened  for  any  new  topic 
or  for  any  expansion  of  an  old  one.  Without  the  consciousness  our  opinions  and 
beliefs  and  practices  undergo  changes;  the  moment  of  our  conversions  could  not  be 
indicated.'  Now  it  seems  that  Dr.  Geikie  proposed  no  radical  doctrine;  his  '  Sugges- 
tions' had  the  face  of  novelty;  to-day  they  are  accepted  as  the  most  correct  and 
natural  from  the  pedagogical  standpoint.     There  is  nothing  radical  about  them. 

"  Briefly,  Dr.  Geikie  possesses  the  widest  knowledge  of  geographical  facts,  and  is 
inspired  with  the  truest  pedagogic  spirit,  and  knows  that  the  knowledge  of  facts 
counts  as  little  unless  gained  in  the  right  way,  that  is,  by  observation  and  induction. 
Further,  to  the  teacher  of  any  subject,  this  brief  treatise  on  geography  will  be  most 
suggestive  in  many  directions. 

"  The  State  Department  of  Education  has  prescribed  this  book  as  one  of  the 
studies  for  the  teachers  of  this  State  in  connection  with  the  summer  normals.  A 
more  excellent  work  could  not  have  been  recommended." —  The  Virginia  School 
yournal. 

ELEMENTARY  LESSONS  IN  PHYSICAL  GEOGRAPHY. 

Illustrated.     i8mo.    $1.10. 

'•  The  language  is  always  simple  and  clear,  and  the  descriptions  of  the  various 
phenomena  are  no  less  vivid  than  interesting;  the  lessons  are  never  dull,  never 
wearisome,  and  they  can  scarcely  fail  to  make  the  study  of  Physical  Geography 
popular  wherever  they  are  used."  —  Academy. 

Questions  on  tlie  Same,  for  Use  in  Schools. 

i8mo.    40  cents. 


GEOGRAPHY  OF  THE  BRITISH  ISLES. 

i8mo.    30  cents. 

"  Dr.  A.  Geikie  is  so  well  known  by  his  able  and  lucid  treatises  on  geology  that 
those  who  believe  in  combining  some  instruction  in  that  branch  of  science  with  the 
teaching  of  geography  will  welcome  a  work  like  the  Elementary  Geography  of  the 
British  Isles,  issued  in  '  Macmillan's  Geographical  Series.'  We  have  rarely  met 
with  a  school  book  at  once  so  delightful  and  so  valuable."  —  Literary  World. 


MACMILLAN   &   CO., 

66   FIFTH   AVENUE,    NEW  YORK. 


THE  BEAUTIES  OF  NATURE 

AND  THE  WONDERS  OF  THE  WORLD  WE  LIVE  IN. 

By  the    Right    Hon.    Sir   JOHN    LUBBOCK,    Bart.,  M.P 

F.R.S.,  D.C.L.,  LL.D., 

Author  of  "  The  Pleasures  of  Life' 

With  Numerous  Illustrations  and  many  Full-page  Plates. 
13mo,  Cloth,  Gilt  Top,   ^1.50. 


"  We  know  of  none  other  better  fitted  to  present  '  the  beauties  ot  nature  and 
the  wonders  of  the  world  we  live  in,'  to  the  popular  understanding  and  appreci- 
ation than  Sir  John  Lubbock,  who  is  at  once  a  master  of  his  chosen  topic  and  of 
a  diction  unsurpassed  for  clearness  and  simplicity  of  statement.  It  is  a  volume 
which  the  reading  public  will  recognize  and  hail  immediately  as  among  the  most 
delightfully  instructive  of  the  year's  production  in  books.  There  is  matter  in 
it  for  the  young  and  the  mature  mind.  .  .  .  One  cannot  rise  from  the  perusal 
of  this  volume,  without  a  consciousness  of  a  mind  invigorated  and  permanently 
enriched  by  an  acquaintance  with  it." —  Osivego  Daily  Times. 

"  It  is  a  charming  book.  .  .  .  Few  writers  succeed  in  making  natural  history, 
and  indeed  scientific  subjects,  more  than  interesting.  In  the  hands  of  most 
authors  they  are  intolerably  dull  to  the  general  reader  and  especially  to  children. 
Sir  John  Lubbock  makes  his  theme  as  entrancing  as  a  novel.  .  .  ,  The  book 
is  magnificently  illustrated,  and  discusses  the  wonders  of  the  animal,  mineral, 
and  vegetable  kingdoms,  the  marvels  of  earth,  sea,  and  the  vaulted  heavens.  In 
the  compass  of  its  pages  an  immense  amount  of  knowledge  which  all  should 
know  is  given  in  a  manner  that  will  compel  the  child  who  commences  it  to 
pursue  it  to  the  end.  It  is  a  work  which  cannot  be  too  highly  recommended 
to  parents  who  have  at  heart  the  proper  education  of  their  children." — The 
Arena. 

"  We  have  here  a  rich  store  of  information  told  in  the  charming  style  for 
which  the  distinguished  author  is  famous.  It  is  suited  alike  to  the  scientific  and 
the  unscientific  reader.  The  wonders  of  animal,  especially  of  the  insect,  life,  of 
plant  life,  of  woods  and  fields,  of  mountains,  of  rivers,  of  lakes,  of  the  .sea  and 
of  the  starry  heavens,  are  here  delightfully  described,  and  they  are  marvellous 
indeed.  ...  It  is  a  good  book  to  kindle  in  the  reader  a  love  of  nature.  .  .  . 
There  is  not  a  dry  or  dull  page  in  the  book."  —  The  Western  Recorder. 

"  We  find  nothing  to  criticise  and  everything  to  enjoy,  .  .  .  The  unpreten- 
tious method  and  the  simplicity  of  the  style  will  attract  even  a  child,  and  the 
whole  book  has  a  winning  power.  .  .  .  The  author  is  copious  in  information, 
suggestive  in  profound  thought,  and  so  clear  and  forcible  in  style  that  man  or 
girl  or  boy  can  enjoy  his  every  page." —  The  Literary  World. 


MACMILLAN    &   CO., 

66  FIFTH    AVENUE,  NEW   YORK. 


THE 

Story  of  the  Hills. 

A    BOOK    ABOUT    MOUNTAINS,    FOR    GENERAL 

READERS. 

By  Rev.  H.  N.  HUTCHINSON,  B.A.,  F.G.S., 

A  nthor  of  "  The  A  uiobiography  of  the  Earth" 
WITH  SIXTEEN  FULL-PAGE  ILLUSTRATIONS. 

12mo.    Cloth  extra.    $1.50. 


'•  Now  that  thousands  of  people  go  every  summer  to  spend  their  holidays 
among  the  mountains,  there  must  be  many  who  would  like  to  know  something 
of  the  secrets  of  the  hills,  —  their  origin,  their  architecture,  and  the  forces  that 
made  them  what  they  are.     For  such  this  book  is  chiefly  written."  —  Preface. 

"  A  most  fascinating  book  for  readers  of  all  ages  and  conditions,  and  espe- 
cially those  addicted  to  travel."  —  School  yournal. 

"...  Mr.  Hutchinson's  graphic  and  entertaining  narrative  concerning  the 
mountains  —  their  origin,  their  architecture,  and  the  forces  that  made  them  what 
they  are  —  has  the  charm  and  interest  of  a  work  of  fiction.  More  wonderful, 
indeed,  is  the  story  unfolded  in  these  pages  than  any  work  of  fiction  could 
possibly  be.  .  .  .  The  volume  is  written  to  suit  the  comprehension  of  the 
ordinary  reader,  and  is  as  free  as  the  subject  will  permit  of  purely  technical 
terms.  The  author  has  brought  the  subject  up  to  date,  so  far  as  geological 
data  and  theories  are  concerned.  The  work  is  profusely  and  beautifully  illus- 
trated with  photographic  views  and  sketches  of  many  famous  mountains."  — 
Christian  at  Work. 

"  A  book  that  has  long  been  needed  is  one  that  shall  give  a  clear  account  of 
the  geological  formation  of  mountains,  and  their  various  methods  of  origin,  in 
language  so  clear  and  technical  that  it  will  not  confuse  even  the  most  unsci- 
entific. Such  a  work  is  that  by  the  Rev.  Mr.  Hutchinson."  —  Boston  Evening 
Transcript. 


MACMILLAN   &   CO., 

66    FIFTH   AVENUE,    NEW   YORK. 


THE  GREAT  WORLD'S  FARM. 

SOME  ACCOUNT   OF  NATURE'S    CROPS   AND    HOW 

THEY  ARE    GROWN. 

By  SELINA   GAYE. 
With  Illustrations.      13mo,  Cloth,  SJl.SO. 


FROM   THE   PREFACE    BY 

G.  S.  BOULGER,  F.L.S.,  F.G.S., 
Professor  of  Botany  and  Geology,  City  of  London  College. 

In  the  attempt,  however,  to  employ  the  teaching  of  science  as  a  means  of 
education;  to  develop,  that  is,  the  innate  mental  faculties  of  a  child,  there  are 
several  dangers  to  which  we  are  exposed ;  we  may,  for  instance,  make  our  sub- 
ject so  uninteresting  that  it  becomes  an  irksome  exercise  of  patience  and  memory, 
and  so  loses  all  its  distinctive  educational  value;  or,  again,  we  may  give  much 
useful  information,  and  even  teach  valuable  lessons  of  observation,  accuracy,  and 
method,  but  fail  to  impart  a  sense  of  proportion,  to  show  the  interdependence  of 
nature  as  a  whole,  or  the  relation  of  our  particular  subject  of  study  to  others  of 
equal  importance. 

Hence  arises  the  great  value  of  books  such  as  the  present,  which,  while  sim- 
ple enough  to  be  understood  by  unscientific  readers,  and  so  accurate  as  to  teach 
nothing  that  will  afterwards  have  to  be  unlearnt,  are  also  extremely  attractive  in 
their  selection  and  marshalling  of  facts. 


PRESS   NOTICES. 

"  It  is  a  book  that  old  and  young  will  learn  much  from,  and  that  by  stimu- 
lating an  interest  in  natural  phenomena  will  lead  to  a  more  systematic  study  of 
the  great  laws  that  govern  the  processes  of  growth  and  transformation  in  the 
material  world  about  us." —  Boston  Beacon. 

"  A  delightful  book.  A  careful  perusal  of  the  volume  excites  surprise  that  so 
large  an  amount  of  scientific  knowledge,  covering  a  great  array  of  points  per- 
tinent to  the  subject,  could  be  presented  in  a  form  that  can  easily  be  grasped  by 
readers  of  ordinary  capacity."  —  N'ew   York  Observer. 

"  The  book  is  filled  full  of  novel  discussions,  of  interesting  facts,  and  of  dis- 
coveries which  could  be  made  only  by  the  closest  study  by  the  enthusiast  in  this 
especial  department  of  research.  With  it  all  there  is  not  a  dull  chapter  or  one 
which  the  reader  will  willingly  leave  unread.^"  —  Boston  Daily  Advertiser. 


MACMILLAN    &    CO., 

66  FIFTH    AVENUE,  NEW   YORK. 


HOURS  IN  MY  GARDEN,  AND  OTHER 
NATURE  SKETCHES. 

By  A.  H.  JAPP. 

With  138  Illustrations  by  W.  H.  J.  Boot,  A.  W.  Cooper,  and 

other  artists. 

Cloth,  S1.75. 


"  A  glance  through  the  pages  of  Dr.  A.  H.  Japp's  '  Hours  in  my  Garden  ' 
leaves  one  with  an  agreeable  impression  of  having  enjoyed  a  summer  ramble  in 
the  country.  The  little  volume  is  made  up  of  nineteen  '  nature  sketches,'  large- 
ly the  fruit  of  personal  observation,  and  it  is  well  freighted  with  the  lighter  lore 
of  the  woods  and  fields,  ponds  and  streams,  hedgerows  and  coppices  of  Old 
England.  The  style  of  the  book  recalls  Richard  Jefferies,  but  there  is  more  lite- 
rary allusion,  and  the  author  has  evidently  looked  at  nature  through  spectacles 
more  scientific  than  poetical.  The  little  essays  are  pleasantly  written,  and  are 
well  adapted  to  stimulate  young  readers  to  a  systematic  study  of  nature.  The 
one  hundred  and  thirty-eight  woodcuts  are  nicely  done,  and  add  to  the  educative 
value  of  the  text."  —  The  Dial. 

"  Lovers  of  nature  have  for  years  past  been  accustomed  to  derive  enjoyment 
and  instruction  from  the  essays  and  field  notes  of  Thoreau,  and,  more  recently, 
John  Burroughs,  Frank  Bolles,  and  two  or  three  others  on  this  side  the  water. 
And  now  comes  an  echo  from  the  other  side  of  the  Atlantic,  in  this  handsome  and 
admirably  illustrated  volume,  telling  us  what  the  author  has  seen  and  deems 
worthy  of  note  in  the  mother  country.  My  Garden  Summer-seat,  My  Pond,  My 
Wood,  Up  in  the  Morning  Early,  The  Village  Well,  A  Scottish  Trout  Stream, 
Wild  Ducks,  Water  Birds,  and  Sea  Fowl  are  the  titles  of  some  of  the  chapters. 
He  is  an  eager  and  careful  observer,  he  finds  much  to  tell  us,  and  his  descriptions 
are  charming  both  in  subject  and  style.  The  author  has  given  reason  to  hope 
'  that  his  essays  may  not  be  found  other  than  pleasant  reading,  and  that  young 
folks  here  and  there  may  derive  some  stimulus  to  more  systematic  study  of  nature 
than  he  was  fortunate  enough  to  have  the  chance  of  making  while  still  young.' 
It  is  a  book  to  be  read  and  enjoyed  by  old  and  young."  —  Public  Opinion. 

"  A  book  that  should  be  read  by  every  one  who  delights  in  the  study  of  nature. 
.  .  .  Bird-lovers  will  be  especially  charmed  with  the  results  of  his  close  intimacy 
with  English  feathered  folk.  He  has  found  wonderful  sagacity  in  the  dainty  den- 
izens of  the  woods  and  fields,  and  he  describes  many  of  their  habits  that  are 
almost  human  in  their  reasonableness  and  wisdom." —  The  Delineator. 

"Mr  Alexander  H.  Japp  has  written  a  series  of  nature  sketches  which  will 
multiply  the  hours  devoted  to  them  to  seasons  of  delight  and  enlarge  the  place  in 
which  they  may  be  read  into  a  garden  beside  which  that  of  Boccaccio  was  a  com- 
mon everyday  field.  He  discourses  about  the  characteristic  features  of  English 
scenery,  cultivated  and  wild,  if  there  can  be  said  to  be  any  wild  scenery  in  Eng- 
land. .  .  .  Mr.  Japp  is  evidently  a  naturalist,  but  he  is  more  than  that,  for  his 
pages  are  instinct  with  feeling  as  well  as  observation,  and  are,  if  one  may  say  so, 
alert  and  alive.  It  is  not  a  book  to  be  described,  but  to  be  read  in  the  spirit  in 
which  it  is  written,  carefully  and  lovingly."  —  Mail  and  Express. 


MACMILLAN    &   CO., 

66  FIFTH    AVENUE,  NEW   YORK. 


MACMILLAN'S 

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ELEMENTARY  PHYSICAL  GEOGRAPHY.  By  Ralph  S.  Takr, 
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Cornell  University,     $1.40. 

LABORATORY  MANUAL  AND  PRINCIPLES  OF  CHEMISTRY  FOR 
BEGINNERS.  By  George  M.  Richardson,  Associate  Professor  of 
Chemistry  in  the  Leland  Stanford  Jr.  University.     $1.10. 

PHYSIOLOGY  FOR  BEGINNERS.  By  Michael  Foster,  M.D.,  F.R.S., 
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LESSONS  IN  APPLIED  MECHANICS.  By  J.  H.  Cotterill  and  J.  H. 
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"  One  of  the  best  little  books  on  the  subject  that  has  come  under  our 
notice  for  some  time."  —  Nature. 

LESSONS  IN  ELEMENTARY  PHYSICS.  By  Professor  Balfour  Stew- 
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"  It  is  the  beau  ideal  of  a  scientific  text-book,  clear,  accurate,  and  thor- 
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EXAMPLES  IN   PHYSICS.     By  Professor  D.  E.  Jones,  B.Sc.     90  cents. 

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lems, and  in  the  solution  of  these  pupils  may  be  trained  to  apply  general 
principles."  —  Joiirnal  of  Education. 

ELEMENTARY    LESSONS    IN    ELECTRICITY   AND    MAGNETISM. 

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exercises  on  the  twelve  chapters  into  which  it  is  divided.  It  is  a  book  which 
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ject."—  Westminster  Reviezv. 

LESSONS  ON  HEAT,  LIGHT,  AND  SOUND.  An  Elementary  Text- 
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ELEMENTARY  LESSONS  ON  ASTRONOMY.  By  J.  N.  Lockyer, 
F.R.S.     With  Illustrations.     $1.25. 

"  The  book  is  full,  clear,  sound,  and  worthy  of  attention,  not  only  as  a 
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14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

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7  DM  USE 

SUMMER 

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LD  21-100m-6,'56 
(B9311sl0)476 


General  Library 

University  of  California 

Berkeley 


'  iw    q^oo  / 


5^425) 


EdjLA       ^' 


^r 


UNIVERSITY  QF  CALIFORNIA  LIBRARY 


